Scientists explore causes of the pause in warming, perhaps the most important research of the decade
Summary: Today we look at papers exploring the pause in warming of the surface atmosphere temperature, now in its second decade. This is one of the world’s most important science priorities, which will guide public policy for the next decade. Do we need a crash program to control CO2 emissions, and mitigate the effects of warming. Or do we have time for more measured responses? There is another dimension to this. Climate science not only teaches us much about our planet, but the reactions of political activists on both says much about modern America.
“The biggest mystery in climate science today…”
— “Climate change: The case of the missing heat“, Jeff Tollefson, Nature, 15 January 2014 — “Sixteen years into the mysterious ‘global-warming hiatus’, scientists are piecing together an explanation.”
“Overdetermination is a phenomenon whereby a single observed effect is determined by multiple causes at once, any one of which alone might be enough to account for (‘determine’) the effect. That is, there are more causes present than are necessary to cause the effect.” (From Wikipedia}
This post is updated as new research becomes available.
- Surveys of the literature
- It’s less water vapor in the atmosphere
- It’s natural variability
- More aerosols (e.g., burn more coal)
- “Stadium Waves”
- It’s the reduced emissions of CFCs and methane
- The Sun
- One temperature dataset mismeasures warming
- Multiple causes, or unknown causes
- Important things to know about global warming
- For More Information
Research into the causes of the pause in surface temperature warming is some of the most important science being done today. We will gain not only vital information about our world, but also — if we look — insights about ourselves that can help us become a more effective society.
(a) Learning about our world
The results of this research might determine public policy priorities for the next decade or more. Do we have time for measured responses to rising CO2 levels, or are crash programs required now to limit them — and prepare for the effects of resumed warming?
Here are some samples of papers about possible causes of the pause. These are all speculative (science takes place on the edges of available data and theory). Eventually these lines of research will produce answers. Meanwhile we get to watch science in action.
(b) Learning about ourselves
American society has long been famously anti-intellectual. The Right cherishes creationists; the Left loves the Pope when he supports their views (He’s a superstitious old reactionary the rest of the time). The climate wars demonstrates that this remains true today.
- Much of the lay debate on both sides consists of personal attacks on climate scientists (guilty of playing for the wrong team). Skeptic websites overflow with attacks on science.
- Climate activist website have abandoned the IPCC as insufficiently alarmist, relying largely on exaggerations of conclusions of outlier papers in the climate literature to paint doomster scenarios as inevitable (the IPCC being created to discourage this).
Now we see the climate activists in full madness. They loudly declare that there is no pause in surface temperature warming. This ignores the dozens of papers mentioning the pause, reports by major climate agencies (e.g., the UK Met Office), papers forecasting when the pause will end, and — the subject of this post — papers about possible causes of the pause.
What’s more pitiful? Activists attempts to conceal the work of climate scientists? Or their followers who close their eyes, seeing only what they’re told to see (a modern version of the prisoners in Plato’s cave).
Now — onto the science. This is a brief summary of research conducted about climate by scientists in many fields. Shown in chronological order. This will be updated as new papers appear. These are given as examples of work on this exciting frontier of climate science. This is not a comprehensive bibliography, nor does it attempt to list all the major papers in each of these areas.
(2) Surveys of the literature
Here are survey papers, reviewing the current state of the various explanations of the pause.
(a) “The recent pause in global warming: What are the potential causes?“, UK Met Office, July 2013 — This is one of a large 3-part paper series by this major climate agency. Excerpt:
There are two main ways to explain the recent surface temperature behaviour; firstly, through changes in the net amount of incoming energy to the climate system (radiative forcing) or, secondly, through redistribution of energy within the climate system, particularly through exchange between the upper and deep ocean, which can temporarily hide the warming below the surface.
Both explanations have been put forward in the literature. For instance, several studies (e.g. Solomon et al 2010, 2011, Church et al 2011) have considered potential radiative forcing explanations, while others (e.g. Knight et al 2009, Meehl et al 2011, Katsman and van Oldenborgh 2011) have examined the potential ocean heat redistribution.
(b) “Heat hide and seek“, Lisa Goddard, Nature Climate Change, March 2014 — “Natural variability can explain fluctuations in surface temperatures but can it account for the current slowdown in warming?”
(c) “Reconciling warming trends“, Gavin A. Schmidt et al, Nature Geoscience, March 2014 — Abstract:
Climate models projected stronger warming over the past 15 years than has been seen in observations. Conspiring factors of errors in volcanic and solar inputs, representations of aerosols, and El Niño evolution, may explain most of the discrepancy.
Conclusion (red emphasis added
We conclude that use of the latest information on external influences on the climate system and adjusting for internal variability associated with ENSO can almost completely reconcile the trends in global mean surface temperature in CMIP5 models and observations. Nevertheless, attributing climate trends over relatively short periods, such as 10 to 15 years, will always be problematic, and it is inherently unsatisfying to find model–data agreement only with the benefit of hindsight. We see no indication, however, that transient climate response is systematically overestimated in the CMIP5 climate models as has been speculated, or that decadal variability across the ensemble of models is systematically underestimated, although at least some individual models probably fall short in this respect.
Most importantly, our analysis implies that significant warming trends are likely to resume, because the dominant long-term warming effect of well-mixed greenhouse gases continues to rise. Asian pollution levels are likely to stabilize and perhaps decrease, although lower solar activity may persist and volcanic eruptions are unpredictable. ENSO will eventually move back into a positive phase and the simultaneous coincidence of multiple cooling effects will cease. Further warming is very likely to be the result.
(3) It’s the reduced level of water vapor in the atmosphere
“Contributions of Stratospheric Water Vapor to Decadal Changes in the Rate of Global Warming“, Susan Solomon et al, Science, 5 March 2010 — Gated. Open copy here. Abstract:
Stratospheric water vapor concentrations decreased by about 10% after the year 2000. Here we show that this acted to slow the rate of increase in global surface temperature over 2000–2009 by about 25% compared to that which would have occurred due only to carbon dioxide and other greenhouse gases. More limited data suggest that stratospheric water vapor probably increased between 1980 and 2000, which would have enhanced the decadal rate of surface warming during the 1990s by about 30% as compared to estimates neglecting this change. These findings show that stratospheric water vapor is an important driver of decadal global surface climate change.
Over the past century, global average surface temperatures have warmed by about 0.75°C. Much of the warming occurred in the past half-century, over which the average decadal rate of change was about 0.13°C, largely due to anthropogenic increases in well-mixed greenhouse gases. However, the trend in global surface temperatures has been nearly flat since the late 1990s despite continuing increases in the forcing due to the sum of the well-mixed greenhouse gases (CO2, CH4, halocarbons, and N2O), raising questions regarding the understanding of forced climate change, its drivers, the parameters that define natural internal variability, and how fully these terms are represented in climate models.
… Figure 3 thus shows that the decline in stratospheric water vapor after 2000 should be expected to have significantly contributed to the flattening of the global warming trend in the past decade, and stratospheric water increases may also have acted to steepen the observed warming trend in the 1990s.
(4) It’s natural variability
(a) “Reconciling anthropogenic climate change with observed temperature 1998–2008“, Robert K. Kaufmann, Heikki Kauppi, Michael L. Mann, and James H. Stock, Proceedings of the National Academy of Sciences (PNAS), 11 July 2011 — Abstract:
Given the widely noted increase in the warming effects of rising greenhouse gas concentrations, it has been unclear why global surface temperatures did not rise between 1998 and 2008. We find that this hiatus in warming coincides with a period of little increase in the sum of anthropogenic and natural forcings. Declining solar insolation as part of a normal eleven-year cycle, and a cyclical change from an El Nino to a La Nina dominate our measure of anthropogenic effects because rapid growth in short-lived sulfur emissions partially offsets rising greenhouse gas concentrations.
As such, we find that recent global temperature records are consistent with the existing understanding of the relationship among global surface temperature, internal variability, and radiative forcing, which includes anthropogenic factors with well known warming and cooling effects.
(b) “Return periods of global climate fluctuations and the pause“, Shaun Lovejoy, Geophysical Research Letters, 16 July 2014 — Open copy here. Abstract:
An approach complementary to General Circulation Models (GCMs), using the anthropogenic CO2 radiative forcing as a linear surrogate for all anthropogenic forcings, was recently developed for quantifying human impacts. Using preindustrial multiproxy series and scaling arguments, the probabilities of natural fluctuations at time lags up to 125 years were determined. The hypothesis that the industrial epoch warming was a giant natural fluctuation was rejected with 99.9% confidence. In this paper, this method is extended to the determination of event return times. Over the period 1880–2013, the largest 32 year event is expected to be 0.47 K, effectively explaining the postwar cooling (amplitude 0.42–0.47 K). Similarly, the “pause” since 1998 (0.28–0.37 K) has a return period of 20–50 years (not so unusual). It is nearly cancelled by the pre-pause warming event (1992–1998, return period 30–40 years); the pause is no more than natural variability.
(c) “Return periods of global climate fluctuations and the pause“, S. Lovejoy, Geophysical Research Letters, 16 July 2014 — Abstract:
An approach complementary to General Circulation Models (GCMs), using the anthropogenic CO2 radiative forcing as a linear surrogate for all anthropogenic forcings [Lovejoy, 2014], was recently developed for quantifying human impacts. Using preindustrial multiproxy series and scaling arguments, the probabilities of natural fluctuations at time lags up to 125 years were determined. The hypothesis that the industrial epoch warming was a giant natural fluctuation was rejected with 99.9% confidence.
In this paper, this method is extended to the determination of event return times. Over the period 1880–2013, the largest 32 year event is expected to be 0.47 K, effectively explaining the postwar cooling (amplitude 0.42–0.47 K). Similarly, the “pause” since 1998 (0.28–0.37 K) has a return period of 20–50 years (not so unusual). It is nearly cancelled by the pre-pause warming event (1992–1998, return period 30–40 years); the pause is no more than natural variability.
(d) “Using data to attribute episodes of warming and cooling in instrumental records“, Ka-Kit Tung and Jiansong Zhou, Proceedings of the National Academy of Sciences (PNAS), 5 February 2013 — Abstract (red emphasis added):
The observed global-warming rate has been nonuniform, and the cause of each episode of slowing in the expected warming rate is the subject of intense debate. To explain this, nonrecurrent events have commonly been invoked for each episode separately. After reviewing evidence in both the latest global data (HadCRUT4) and the longest instrumental record, Central England Temperature, a revised picture is emerging that gives a consistent attribution for each multidecadal episode of warming and cooling in recent history, and suggests that the anthropogenic global warming trends might have been overestimated by a factor of two in the second half of the 20th century.
A recurrent multidecadal oscillation is found to extend to the preindustrial era in the 353-y Central England Temperature and is likely an internal variability related to the Atlantic Multidecadal Oscillation (AMO), possibly caused by the thermohaline circulation variability. The perspective of a long record helps in quantifying the contribution from internal variability, especially one with a period so long that it is often confused with secular trends in shorter records. Solar contribution is found to be minimal for the second half of the 20th century and less than 10% for the first half.
The underlying net anthropogenic warming rate in the industrial era is found to have been steady since 1910 at 0.07–0.08 °C/decade, with superimposed AMO-related ups and downs that included the early 20th century warming, the cooling of the 1960s and 1970s, the accelerated warming of the 1980s and 1990s, and the recent slowing of the warming rates. Quantitatively, the recurrent multidecadal internal variability, often underestimated in attribution studies, accounts for 40% of the observed recent 50-y warming trend.
(a) See 4(a) above
“… rapid growth in short-lived sulfur emissions partially offsets rising greenhouse gas concentrations”
(b) Contrary analysis: “Climate impacts of changing aerosol emissions since 1996“, T Kuhn et al, Geophysical Research Letters, 16 jULY 2014 — Gated. Abstract (red emphasis added):
Increases in Asian aerosol emissions have been suggested as one possible reason for the hiatus in global temperature increase during the past 15 years. We study the effect of sulphur and black carbon (BC) emission changes between 1996 and 2010 on the global energy balance. We find that the increased Asian emissions have had very little regional or global effects, while the emission reductions in Europe and the U.S. have caused a positive radiative forcing. In our simulations, the global-mean aerosol direct radiative effect changes by 0.06 W/m2 during 1996 to 2010, while the effective radiative forcing (ERF) is 0.42 W/m2. The rather large ERF arises mainly from changes in cloudiness, especially in Europe. In Asia, the BC warming due to sunlight absorption has largely offset the cooling caused by sulphate aerosols. Asian BC concentrations have increased by a nearly constant fraction at all altitudes, and thus, they warm the atmosphere also in cloudy conditions.
(c) “Contribution of anthropogenic sulfate aerosols to the changing Euro-Mediterranean climate since 1980“, Pierre Nabat, Geophysical Research Letters, 16 August 2014 — Abstract:
Since the 1980s anthropogenic aerosols have been considerably reduced in Europe and the Mediterranean area. This decrease is often considered as the likely cause of the brightening effect observed over the same period. This phenomenon is however hardly reproduced by global and regional climate models.
Here we use an original approach based on reanalysis-driven coupled regional climate system modeling to show that aerosol changes explain 81 ± 16% of the brightening and 23 ± 5% of the surface warming simulated for the period 1980–2012 over Europe. The direct aerosol effect is found to dominate in the magnitude of the simulated brightening. The comparison between regional simulations and homogenized ground-based observations reveals that observed surface solar radiation and land and sea surface temperature spatiotemporal variations over the Euro-Mediterranean region are only reproduced when simulations include the realistic aerosol variations.
(6) It’s the volcanoes
(a) “Recent anthropogenic increases in SO2 from Asia have minimal impact on stratospheric aerosol“, Ryan Neely et al, Geophysical Research Letters, 13 March 2013 — Pdf of slide presentation. Abstract:
Observations suggest that the optical depth of the stratospheric aerosol layer between 20 and 30 km has increased 4–10% per year since 2000, which is significant for Earth’s climate. Contributions to this increase both from moderate volcanic eruptions and from enhanced coal burning in Asia have been suggested. Current observations are insufficient to attribute the contribution of the different sources. Here we use a global climate model coupled to an aerosol microphysical model to partition the contribution of each.
We employ model runs that include the increases in anthropogenic sulfur dioxide (SO2) over Asia and the moderate volcanic explosive injections of SO2 observed from 2000 to 2010. Comparison of the model results to observations reveals that moderate volcanic eruptions, rather than anthropogenic influences, are the primary source of the observed increases in stratospheric aerosol.
(b) “Volcanic contribution to decadal changes in tropospheric temperature“, Benjamin D. Santer et al, Nature Geoscience, March 2014 — Gated. Abstract:
Despite continued growth in atmospheric levels of greenhouse gases, global mean surface and tropospheric temperatures have shown slower warming since 1998 than previously. Possible explanations for the slow-down include internal climate variability, external cooling influences and observational errors. Several recent modelling studies have examined the contribution of early twenty-first-century volcanic eruptions to the muted surface warming.
Here we present a detailed analysis of the impact of recent volcanic forcing on tropospheric temperature, based on observations as well as climate model simulations. We identify statistically significant correlations between observations of stratospheric aerosol optical depth and satellite-based estimates of both tropospheric temperature and short-wave fluxes at the top of the atmosphere.
We show that climate model simulations without the effects of early twenty-first-century volcanic eruptions overestimate the tropospheric warming observed since 1998. In two simulations with more realistic volcanic influences following the 1991 Pinatubo eruption, differences between simulated and observed tropospheric temperature trends over the period 1998 to 2012 are up to 15% smaller, with large uncertainties in the magnitude of the effect. To reduce these uncertainties, better observations of eruption-specific properties of volcanic aerosols are needed, as well as improved representation of these eruption-specific properties in climate model simulations.
(c) “Total volcanic stratospheric aerosol optical depths and implications for global climate change“, David Ridley et al, Geophysical Research Letters, in press — Abstract:
Understanding the cooling effect of recent volcanoes is of particular interest in the context of the post-2000 slowing of the rate of global warming. Satellite observations of aerosol optical depth (AOD) above 15 km have demonstrated that small-magnitude volcanic eruptions substantially perturb incoming solar radiation. Here we use lidar, AERONET and balloon-borne observations to provide evidence that currently available satellite databases neglect substantial amounts of volcanic aerosol between the tropopause and 15 km at mid to high latitudes, and therefore underestimate total radiative forcing resulting from the recent eruptions. Incorporating these estimates into a simple climate model, we determine the global volcanic aerosol forcing since 2000 to be −0.19 ± 0.09 Wm−2. This translates into an estimated global cooling of 0.05 to 0.12 °C. We conclude that recent volcanic events are responsible for more post-2000 cooling than is implied by satellite databases that neglect volcanic aerosol effects below 15 km.
The eminent Roger Pielke Sr (see Wikipedia) has long said that the focus on the surface air temperature was inappropriate, and that…
“The spatial pattern of ocean heat content change is the appropriate metric to assess climate system heat changes including global warming.” (source)
For this he was smeared, and called a denier by activists. Such as those at Skeptical Science (Dana Nuccitelli’s launch pad, which should be called “skeptical of science”). See this page calling him a “climate misinformer” (note that all of his quotes shown there now appear correct). See this note for a more detail, and references to his work.
Now that the surface temperature has paused, climate scientists have realized that he was correct. And of course activists give no retraction or apologies for their smears. Now back to the scientists …
(a) “An apparent hiatus in global warming?“, Kevin E. Trenberth and John T. Fasullo, Earth’s Future, 5 December 2013 — Also see Trenberth’s “Has Global Warming Stalled” at The Conversation, 23 May 2014. Abstract:
Global warming first became evident beyond the bounds of natural variability in the 1970s, but increases in global mean surface temperatures have stalled in the 2000s. Increases in atmospheric greenhouse gases, notably carbon dioxide, create an energy imbalance at the top-of-atmosphere (TOA) even as the planet warms to adjust to this imbalance, which is estimated to be 0.5–1 W m−2 over the 2000s.
Annual global fluctuations in TOA energy of up to 0.2 W m−2 occur from natural variations in clouds, aerosols, and changes in the Sun. At times of major volcanic eruptions the effects can be much larger. Yet global mean surface temperatures fluctuate much more than these can account for. An energy imbalance is manifested not just as surface atmospheric or ground warming but also as melting sea and land ice, and heating of the oceans.
More than 90% of the heat goes into the oceans and, with melting land ice, causes sea level to rise. For the past decade, more than 30% of the heat has apparently penetrated below 700 m depth that is traceable to changes in surface winds mainly over the Pacific in association with a switch to a negative phase of the Pacific Decadal Oscillation (PDO) in 1999. Surface warming was much more in evidence during the 1976–1998 positive phase of the PDO, suggesting that natural decadal variability modulates the rate of change of global surface temperatures while sea-level rise is more relentless.
Global warming has not stopped; it is merely manifested in different ways.
(b) “Sixteen years into the mysterious ‘global-warming hiatus’, scientists are piecing together an explanation.“, Jeff Tollefson, Nature, 15 January 2014 — Well-written news feature; not peer-reviewed research. Opening:
The biggest mystery in climate science today may have begun, unbeknownst to anybody at the time, with a subtle weakening of the tropical trade winds blowing across the Pacific Ocean in late 1997. These winds normally push sun-baked water towards Indonesia. When they slackened, the warm water sloshed back towards South America, resulting in a spectacular example of a phenomenon known as El Niño. Average global temperatures hit a record high in 1998 — and then the warming stalled.
… Now, as the global-warming hiatus enters its 16th year, scientists are at last making headway in the case of the missing heat. Some have pointed to the Sun, volcanoes and even pollution from China as potential culprits, but recent studies suggest that the oceans are key to explaining the anomaly. The latest suspect is the El Niño of 1997–98, which pumped prodigious quantities of heat out of the oceans and into the atmosphere — perhaps enough to tip the equatorial Pacific into a prolonged cold state that has suppressed global temperatures ever since.
This is a remarkable article, especially in one of the premier science journals. The role of ocean-atmosphere cycles (e.g., PDO, AMO, ENSO) in warming has been a major theme of climate skeptics — of which Bob Tisdale is the best-known. Activists have smeared and mocked them, so recognition of the validity of their insights is a stunning vindication, even if uncredited in this article. This is a common pattern in the history of science, professionals following path-breaking work by amateurs.
(c) It’s El Nino
“The influence of different El Niño types on global average temperature“, Sandra Banholzer and Simon Donner, Geophysical Research Letters, 28 March 2014 — ScienceDaily article here. Abstract:
The El Niño – Southern Oscillation is known to influence surface temperatures worldwide. El Niño conditions are thought to lead to anomalously warm global average surface temperature, absent other forcings. Recent research has identified distinct possible types of El Niño events based on the location of peak sea surface temperature anomalies.
Here we analyze the relationship between the type of El Niño event and the global surface average temperature anomaly, using three historical temperature data sets. Separating El Niño events into types reveals that the global average surface temperatures are anomalously warm during and after traditional eastern Pacific El Niño events, but not central Pacific or mixed events. Historical analysis indicated that slowdowns in the rate of global surface warming since the late 1800s may be related to decadal variability in the frequency of different types of El Niño events.
(d) For a review of the literature on changes in the oceans’ heat content see this article by Judith Curry (Prof Climate Science, GA Institute of Tech). She points to this new paper (Journal of Climate, in press) describing the large uncertainties in measurements of the ocean heat content.
The globally integrated heat content changes involve small differences of the much larger regional changes. As existing estimates of the anthropogenic forcing are now about 0.5W/m2, the equivalent global ocean average temperature changes over 20 years are mostly slight compared to the shorter term temporal variations from numerous physical sources. Detailed attention must be paid to what might otherwise appear to be small errors in data calibration, and space-time sampling and model biases. Direct determination of changes in oceanic heat content over the last 20 years are not in conflict with estimates of the radiative forcing, but the uncertainties remain too large to rationalize e.g., the apparent “pause” in warming.
(f) “Varying planetary heat sink led to global-warming slowdown and acceleration“, Xianyao Chen and Ka-Kit Tung, Science, 22 August 2014 — Abstract:
A vacillating global heat sink at intermediate ocean depths is associated with different climate regimes of surface warming under anthropogenic forcing: The latter part of the 20th century saw rapid global warming as more heat stayed near the surface. In the 21st century, surface warming slowed as more heat moved into deeper oceans. In situ and reanalyzed data are used to trace the pathways of ocean heat uptake. In addition to the shallow La Niña–like patterns in the Pacific that were the previous focus, we found that the slowdown is mainly caused by heat transported to deeper layers in the Atlantic and the Southern oceans, initiated by a recurrent salinity anomaly in the subpolar North Atlantic. Cooling periods associated with the latter deeper heat-sequestration mechanism historically lasted 20 to 35 years.
(g) “Surface warming hiatus caused by increased heat uptake across multiple ocean basins“, S. S. Drijfhout et al, Geophysical Research Letters, in press — Abstract:
The first decade of the twenty-first century was characterised by a hiatus in global surface warming. Using ocean model hindcasts and reanalyses we show that heat uptake between the 1990s and 2000s increased by 0.7 ± 0.3Wm−2. Approximately 30% of the increase is associated with colder sea surface temperatures in the eastern Pacific. Other basins contribute via reduced heat loss to the atmosphere, in particular the Southern and subtropical Indian Oceans (30%), and the subpolar North Atlantic (40%). A different mechanism is important at longer timescales (1960s-present) over which the Southern Annular Mode trended upwards. In this period, increased ocean heat uptake has largely arisen from reduced heat loss associated with reduced winds over the Agulhas Return Current and southward displacement of Southern Ocean westerlies.
(8) “Stadium Waves”
“Role for Eurasian Arctic shelf sea ice in a secularly varying hemispheric climate signal during the 20th century”, Marcia Glaze Wyatt and Judith A. Curry, Climate Dynamics, September 2013 — Gated. Open copy here. See Curry’s discussion of the paper here. Abstract:
A hypothesized low-frequency climate signal propagating across the Northern Hemisphere through a network of synchronized climate indices was identified in previous analyses of instrumental and proxy data. The tempo of signal propagation is rationalized in terms of the multidecadal component of Atlantic Ocean variability — the Atlantic Multidecadal Oscillation. Through multivariate statistical analysis of an expanded database, we further investigate this hypothesized signal to elucidate propagation dynamics.
The Eurasian Arctic Shelf-Sea Region, where sea ice is uniquely exposed to open ocean in the Northern Hemisphere, emerges as a strong contender for generating and sustaining propagation of the hemispheric signal. Ocean-ice-atmosphere coupling spawns a sequence of positive and negative feedbacks that convey persistence and quasi-oscillatory features to the signal. Further stabilizing the system are anomalies of co-varying Pacific-centered atmospheric circulations.
Indirectly related to dynamics in the Eurasian Arctic, these anomalies appear to negatively feed back onto the Atlantic‘s freshwater balance. Earth’s rotational rate and other proxies encode traces of this signal as it makes its way across the Northern Hemisphere.
(9) Its the reduced emissions of CFCs and methane
“Statistically derived contributions of diverse human influences to twentieth-century temperature changes“, Francisco Estrada, Pierre Perron and Benjamín Martínez-López, Nature Geoscience, December 2013 — Abstract:
The warming of the climate system is unequivocal as evidenced by an increase in global temperatures by 0.8 °C over the past century. However, the attribution of the observed warming to human activities remains less clear, particularly because of the apparent slow-down in warming since the late 1990s.
Here we analyse radiative forcing and temperature time series with state-of-the-art statistical methods to address this question without climate model simulations. We show that long-term trends in total radiative forcing and temperatures have largely been determined by atmospheric greenhouse gas concentrations, and modulated by other radiative factors. We identify a pronounced increase in the growth rates of both temperatures and radiative forcing around 1960, which marks the onset of sustained global warming.
Our analyses also reveal a contribution of human interventions to two periods when global warming slowed down. Our statistical analysis suggests that the reduction in the emissions of ozone-depleting substances under the Montreal Protocol, as well as a reduction in methane emissions, contributed to the lower rate of warming since the 1990s. Furthermore, we identify a contribution from the two world wars and the Great Depression to the documented cooling in the mid-twentieth century, through lower carbon dioxide emissions.
We conclude that reductions in greenhouse gas emissions are effective in slowing the rate of warming in the short term.
There is a large and growing body of research on this topic. Here is a recent paper about the effects of the ozone: “Climate System Response to Stratospheric Ozone Depletion and Recovery“, Michael Previdi and Lorenzo M. Polvan, Quarterly Journal of the Royal Meteorological Society, in press (gated; open copy here).
(10) It’s the reduced solar activity
Many scientists have pointed to changes in solar activity as drivers of decadal or century-long climate cycles (see research listed in section #7 here). Climate activists have denounced as “deniers” anyone daring to mention this research (see the comments to the posts about solar cycles). Now that’s gone down the memory hole, as solar activity becomes an explanation for the pause (we’ve always been at war with EastAsia). Back to the science…
“Reduced Solar Activity Disguises Global Temperature Rise“, Peter Stauning, Atmosphere and Climate Sciences, January 2014 – Abstract:
The question whether human activities seriously affect climate is asked with increasing voice these days. Quite understandable since the climate appears to be out of control with the significant global temperature increases already seen during the last 3 decades and with still heavier temperature increases to come in the future according to prognoses, among others, in the recent comprehensive IPCC reports.
However, the most recent climate data, show global temperature development levelling off or even turning negative since 2001 in contrast to the anticipated course related to the steady increases in the concentration in the atmosphere of green-house gasses, primarily carbon dioxide and methane.
The purpose of this communication is to demonstrate that the reduced rate in the global temperature rise complies with expectations related to the decaying level of solar activity according to the relation published in an earlier analysis. Without the reduction in the solar activity-related contributions the global temperatures would have increased steadily from 1980 to present.
(11) One of the 4 major global temperature datasets underestimates warming
Doubting the accuracy of global temperature measurements has been considered heresy by climate activists (denier, denier they’d cry). But desperate for salvation they greeted this paper with euphoric celebration, although much of the initial response by climate scientists was critical (examples here). Plus, of course, there are three other major temperature datasets.
“Coverage bias in the HadCRUT4 temperature series and its impact on recent temperature trends“, Kevin Cowtan and Robert G. Way, Quarterly Journal of the Royal Meteorology Society, in press — Abstract:
In complete global coverage is a potential source of bias in global temperature reconstructions if the unsampled regions are not uniformly distributed over the planet’s surface. The widely used HadCRUT4 dataset covers on average about 84% of the globe over recent decades, with the unsampled regions being concentrated at the poles and over Africa.
Three existing reconstructions with near-global coverage are examined, each suggesting that HadCRUT4 is subject to bias due to its treatment of unobserved regions. Two alternative approaches for reconstructing global temperatures are explored, one based on an optimal interpolation algorithm and the other a hybrid method incorporating additional information from the satellite temperature record. The methods are validated on the basis of their skill at reconstructing omitted sets of observations. Both methods provide superior results than excluding the unsampled regions, with the hybrid method showing particular skill around the regions where no observations are available.
Temperature trends are compared for the hybrid global temperature reconstruction and the raw HadCRUT4 data. The widely quoted trend since 1997 in the hybrid global reconstruction is two and a half times greater than the corresponding trend in the coverage-biased HadCRUT4 data. Coverage bias causes a cool bias in recent temperatures relative to the late 1990s which increases from around 1998 to the present. Trends starting in 1997 or 1998 are particularly biased with respect to the global trend. The issue is exacerbated by the strong El Ni ̃no event of 1997-1998, which also tends to suppress trends starting during those years.
(a) The trade winds have changed: “Recent intensification of wind-driven circulation in the Pacific and the ongoing warming hiatus“, Matthew H. England et al, Nature Climate Change, March 2014 — Gated. Abstract:
Despite ongoing increases in atmospheric greenhouse gases, the Earth’s global average surface air temperature has remained more or less steady since 2001. A variety of mechanisms have been proposed to account for this slowdown in surface warming. A key component of the global hiatus that has been identified is cool eastern Pacific sea surface temperature, but it is unclear how the ocean has remained relatively cool there in spite of ongoing increases in radiative forcing.
Here we show that a pronounced strengthening in Pacific trade winds over the past two decades — unprecedented in observations/reanalysis data and not captured by climate models — is sufficient to account for the cooling of the tropical Pacific and a substantial slowdown in surface warming through increased subsurface ocean heat uptake. The extra uptake has come about through increased subduction in the Pacific shallow overturning cells, enhancing heat convergence in the equatorial thermocline.
At the same time, the accelerated trade winds have increased equatorial upwelling in the central and eastern Pacific, lowering sea surface temperature there, which drives further cooling in other regions.
The net effect of these anomalous winds is a cooling in the 2012 global average surface air temperature of 0.1–0.2 °C, which can account for much of the hiatus in surface warming observed since 2001. This hiatus could persist for much of the present decade if the trade wind trends continue, however rapid warming is expected to resume once the anomalous wind trends abate.
Their work is part of the ongoing shift of climate scientists’ expectations away from the forecasts of extreme warming. Excerpt:
… model is equilibrated for more than 3,000 years with atmospheric CO2 fixed at pre-industrial and then integrated during 1780 – 2030 following historical CO2 forcing (1780 – 2000) and then the CMIP3 A2 emissions scenario 46 from the year 2000 onwards. The model’s overall climate sensitivity is at the low end range of CMIP3 models (reaching 2.1 C warming by 2090 – 2099 relative to 1980 – 1999); …
(b) “Atmospheric science: Increasing wind sinks heat“, Yu Kosaka, Nature Climate Change, March 2014 — Abstract:
Surface global warming has stalled since around 2000 despite increasing atmospheric CO2. A study finds that recent strengthening of Pacific trade winds has enhanced heat transport from the surface to ocean depths, explaining most of the slowed surface warming.
(c) “Contribution of natural decadal variability to global warming acceleration and hiatus“, Masahiro Watanabe et al, Nature Climate Change, in press — Abstract:
Reasons for the apparent pause in the rise of global-mean surface air temperature (SAT) after the turn of the century has been a mystery, undermining confidence in climate projections. Recent climate model simulations indicate this warming hiatus originated from eastern equatorial Pacific cooling associated with strengthening of trade winds. Using a climate model that overrides tropical wind stress anomalies with observations for 1958–2012, we show that decadal-mean anomalies of global SAT referenced to the period 1961–1990 are changed by 0.11, 0.13 and −0.11 °C in the 1980s, 1990s and 2000s, respectively, without variation in human-induced radiative forcing. They account for about 47%, 38% and 27% of the respective temperature change.
The dominant wind stress variability consistent with this warming/cooling represents the deceleration/acceleration of the Pacific trade winds, which can be robustly reproduced by atmospheric model simulations forced by observed sea surface temperature excluding anthropogenic warming components. Results indicate that inherent decadal climate variability contributes considerably to the observed global-mean SAT time series, but that its influence on decadal-mean SAT has gradually decreased relative to the rising anthropogenic warming signal.
“Late Twentieth-Century Warming and Variations in Cloud Cover“, John McLean, Atmospheric and Climate Sciences, October 2014 — For an explanation of the significance of this paper see “Climate dynamics of clouds” by Judith Curry.
(14) Multiple causes, or unknown causes
(a) “Drivers of decadal hiatus periods in the 20th and 21st centuries“, Matthew H. England et al, Geophysical Research Letters, 28 August 2014 — Abstract:
The latest generation of climate model simulations are used to investigate the occurrence of hiatus periods in global surface air temperature in the past and under two future warming scenarios. Hiatus periods are identified in three categories:
- those due to volcanic eruptions,
- those associated with negative phases of the Interdecadal Pacific Oscillation (IPO), and
- those affected by anthropogenically released aerosols in the mid-twentieth century.
The likelihood of future hiatus periods is found to be sensitive to the rate of change of anthropogenic forcing. Under high rates of greenhouse gas emissions there is little chance of a hiatus decade occurring beyond 2030, even in the event of a large volcanic eruption. We further demonstrate that most nonvolcanic hiatuses across Coupled Model Intercomparison Project 5 (CMIP5) models are associated with enhanced cooling in the equatorial Pacific linked to the transition to a negative IPO phase.
(b) “Natural variability, radiative forcing and climate response in the recent hiatus reconciled“, Markus Huber and Reto Knutti, Nature Geoscience, September 2014 — Gated. Abstract:
Global mean surface warming over the past 15 years or so has been less than in earlier decades and than simulated by most climate models1. Natural variability, a reduced radiative forcing5, a smaller warming response to atmospheric carbon dioxide concentrations and coverage bias in the observations10 have been identified as potential causes. However, the explanations of the so-called ‘warming hiatus’ remain fragmented and the implications for long-term temperature projections are unclear. Here we estimate the contribution of internal variability associated with the El Niño/Southern Oscillation (ENSO) using segments of unforced climate model control simulations that match the observed climate variability. We find that ENSO variability analogous to that between 1997 or 1998 and 2012 leads to a cooling trend of about −0.06 °C.
In addition, updated solar and stratospheric aerosol forcings from observations explain a cooling trend of similar magnitude (−0.07 °C). Accounting for these adjusted trends we show that a climate model of reduced complexity with a transient climate response of about 1.8 °C is consistent with the temperature record of the past 15 years, as is the ensemble mean of the models in the Coupled Model Intercomparison Project Phase 5 (CMIP5). We conclude that there is little evidence for a systematic overestimation of the temperature response to increasing atmospheric CO2 concentrations in the CMIP5 ensemble.
(c) “Application of the Singular Spectrum Analysis Technique to Study the Recent Hiatus on the Global Surface Temperature Record“, Diego Macias et al, PLOS, 10 September 2014 — See ScienceBlog for an explanation of this study. Abstract:
Global surface temperature has been increasing since the beginning of the 20th century but with a highly variable warming rate, and the alternation of rapid warming periods with ‘hiatus’ decades is a constant throughout the series. The superimposition of a secular warming trend with natural multidecadal variability is the most accepted explanation for such a pattern.
Since the start of the 21st century, the surface global mean temperature has not risen at the same rate as the top-of-atmosphere radiative energy input or greenhouse gas emissions, provoking scientific and social interest in determining the causes of this apparent discrepancy. Multidecadal natural variability is the most commonly proposed cause for the present hiatus period.
Here, we analyze the HadCRUT4 surface temperature database with spectral techniques to separate a multidecadal oscillation (MDV) from a secular trend (ST). Both signals combined account for nearly 88% of the total variability of the temperature series showing the main acceleration/deceleration periods already described elsewhere. Three stalling periods with very little warming could be found within the series, from 1878 to 1907, from 1945 to 1969 and from 2001 to the end of the series, all of them coincided with a cooling phase of the MDV. Henceforth, MDV seems to be the main cause of the different hiatus periods shown by the global surface temperature records. However, and contrary to the two previous events, during the current hiatus period, the ST shows a strong fluctuation on the warming rate, with a large acceleration (0.0085°C year−1 to 0.017°C year−1) during 1992–2001 and a sharp deceleration (0.017°C year−1 to 0.003°C year−1) from 2002 onwards.
This is the first time in the observational record that the ST shows such variability, so determining the causes and consequences of this change of behavior needs to be addressed by the scientific community.
(15) A few important things to remember about global warming
While cheering for their faction of scientists, laypeople often lose sight of the big picture — the key elements for making public policy about this important issue. See the details here.
(16) For More Information
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