One of the most important questions we face: when will the pause in global warming end?

Summary:  Global warming has paused since roughly 1998 (see links in section 10). Today we examine climate scientists’ forecasts of when it will end (spoiler: now, or a decade-plus from now).

Contents

1. One of the first to see the pause.
2. Professor Mojib Latif.
3. Barrie G. Hunt (CSIRO).
4. James Hansen (NASA).
5. Kevin Trenberth (NOAA).
6. Professor Judith Curry.
7. UK Met Office Decadal Forecast.
8. UK Met Office analysis of the pause.
9. John Fyfe et al.
10. Marcia Glaze Wyatt & Judith A. Curry.
11. Jianping Li.
12. James Hansen et al.
13. Matthew H. England et al.
14. Xianyao Chen and Ka-Kit Tung.
15. C. D. Roberts et al.
16. Steven Smith et al.
17. Gerard D. McCarthy et al.
18. Knutson et al.

This post will be updated as new research is published. These are only a sample of the research in this exciting and important area. Graphic from the September 2013 National Geographic.

Introduction

The duration of the pause in global warming will make no difference to the long-term history of the planet, and probably little difference to 21st century climate trends. But it might have large political impact, determining the magnitude of our preparations for our changing climate. That’s bad, since both climate research and preparations are absurdly underfunded — inadequate for even normal weather (NYT is at sea level; Sandy revealed it had near-zero flood preparations). The next in this series will discuss the politics of the pause.

Research

(1)  A so-far successful prediction: One of the first papers reporting the pause:  “Advancing decadal-scale climate prediction in the North Atlantic sector“, N. S. Keenlyside, M. Latif, J. Jungclaus, L. Kornblueh & E. Roeckner, Nature, 1 May 2008 — Conclusion of the abstract:

Our results suggest that global surface temperature may not increase over the next decade, as natural climate variations in the North Atlantic and tropical Pacific temporarily offset the projected anthropogenic warming.

Note they say “temporarily”. In the paper they show warming resuming, then continuing through the end of their forecast period in 2030. For more information see the BBC’s article about it and the authors’ slides.

Four years later, their prediction has already been proven false

“How will Earth’s surface temperature change in future decades?“, Judith L. Lean and David H. Rind, Geophysical Research Letters, August 2009 — Abstract (red emphasis added):

Reliable forecasts of climate change in the immediate future are difficult, especially on regional scales, where natural climate variations may amplify or mitigate anthropogenic warming in ways that numerical models capture poorly. By decomposing recent observed surface temperatures into components associated with ENSO, volcanic and solar activity, and anthropogenic influences, we anticipate global and regional changes in the next two decades.

1. From 2009 to 2014, projected rises in anthropogenic influences and solar irradiance will increase global surface temperature 0.15 ± 0.03°C, at a rate 50% greater than predicted by IPCC.
2. But as a result of declining solar activity in the subsequent five years, average temperature in 2019 is only 0.03 ± 0.01°C warmer than in 2014.

This lack of overall warming is analogous to the period from 2002 to 2008 when decreasing solar irradiance also countered much of the anthropogenic warming. We further illustrate how a major volcanic eruption and a super ENSO would modify our global and regional temperature projections.

Other excerpts:

Yet as Figure 1 shows, global surface temperatures warmed little, if at all, from 2002 to 2008, even as green- house gas concentrations have increased, causing some to question the reality of anthropogenic global warming.

… in time scales of 10 to 50 years (and longer) decadal climate forecasts are difficult to make with general circulation climate models due to their many uncertainties [IPCC , 2007].

(2)  Presentation by Mojib Latif (Prof Climate Physics, Kiel University) at the World Climate Conference #3, 1 September 2009 — Slides here; recording here. Note the graph does not distinguish between actual past data and forecastes.

“it may well happen that you enter a decade, or maybe even two, when the temperature cools relative to the present level.”

(3)  “The role of natural climatic variation in perturbing the observed global mean temperature trend“, Barrie G Hunt (Australia’s Commonwealth Scientific and Industrial Research Organisation), Climate Dynamics, February 2011 — Gated. Abstract:

The characteristics of the internally-induced negative temperature anomalies are such that if this internal natural variability is the cause of the observed hiatus, then a resumption of the observed global warming trend is to be expected within the next few years.

(4)  “Global Temperature Update Through 2012“, James Hansen (NASA), M. Sato, R. Ruedy, 15 January 2013 — Excerpt:

 

… the continuing planetary energy imbalance and the rapid increase of CO 2 emissions from fossil fuel use assure that global warming will continue on decadal time scales. Moreover, our interpretation of the larger role of unforced variability in temperature change of the past decade, suggests that global temperature will rise significantly in the next few years as the tropics moves inevitably into the next El Nino phase.

The one major wild card in projections of fut ure climate change is the unmeasured climate forcing due to aerosol changes and their effects on clouds. Anecdotal information indicates that particulate air pollution has increased in regions with increasing coal burning, but assessment of the climate forcing requires global measurement of detailed physical properties of the aerosols. The one satellite mission that was capable of making measurements with the required detail and accuracy was lost via a launch failure, and as yet there are no plans for a replacement mission with the needed capabilities.

(5)  Transcript of interview with Kevin Trenberth (Senior Scientist, National Center for Atmospheric Research; bio here), All Things Considered, NPR, 23 August 2013

HARRIS: Trenberth says the planet has been heating up during that time, it’s just that the heat has been flowing into the oceans, which have a vast capacity to absorb it. Will the oceans come to our rescue, essentially?

TRENBERTH: That’s a good question, and the answer is maybe partly yes, but maybe partly no.

HARRIS: The oceans can, at times, soak up a lot of heat. Some of it goes into the deep oceans, where it can stay for centuries. But heat absorbed closer to the surface can easily flow back into the air. That happened in 1998, which made it one of the hottest years on record. Trenberth says since then, the ocean has mostly been back in one of its soaking-up phases.

TRENBERTH: They probably can’t go on much for much longer than maybe 20 years, and what happens at the end of these hiatus periods is that suddenly there’s a big jump to a whole new level and you never go back down to that previous level again.

(6)  Statement of Judith Curry (Prof climate science, GA Institute of Technology; bio here) to the Subcommittee on Environment on 25 April 2013 — Excerpt:

In light of these uncertainties, what can we say about the future climate of the 21st century? Most scientists anticipate a decrease in solar forcing in the coming decades, but noting the absence of understanding the solar indirect effects on climate, this is not expected to dominate climate change in the 21st century. If the climate shifts hypothesis is correct, then the current flat trend in global surface temperatures may continue for another decade or two, with a resumption of warming at some point during mid-century. The amount of warming from greenhouse gases depends both on the amount of greenhouse gases that are emitted as well as the climate sensitivity to the greenhouse gases, both of which are associated with substantial uncertainties.

(7)  Decadal Forecast, UK Met Office, December 2012 — Forecast of flat, carefully phrased.

… we will continue to see temperatures like those which resulted in 2000-2009 being the warmest decade in the instrumental record dating back to 1850. (Update, 8 January 2013)

Observed (black, from Hadley Centre, GISS and NCDC) and predicted global average annual surface temperature difference relative to 1971-2000. Retrospective predictions starting from June 1960, 1965, …, 2005 are shown as white curves, with red shading representing their probable range, such that the observations are expected to lie within the shading 90% of the time. The most recent forecast (thick blue curve with thin blue curves showing range) starts from November 2012. All data are rolling annual mean values. The gap between the black and blue curves arises because the last observed value represents the period November 2011 to October 2012 whereas the first forecast period is November 2012 to October 2013.

(8)  “The recent pause in global warming, part 3: What are the implications for projections of future warming?“, UK Met Office, July 2013 — TCR: transient climate response. ECS: equilibrium climate sensitivity. Excerpt:

When projections from the newer climate models are combined with observations, including those from the last 10 years, the uncertainty range for warming out to 2050 is reduced. The very highest values of projected warming are eliminated, but the lower bound is largely unchanged. The most likely warming is reduced by only 10%, indicating that the warming that we might previously have expected by 2050 would be delayed by only a few years.

… ECS can be estimated from a range of methods: directly from observations, from comprehensive climate models or by combining models with observations. ECS can also be estimated using palaeoclimate reconstructions of periods in the distant past, such as the last glacial maximum. Again there are questions of accuracy and the comprehensiveness of palaeoclimate records.

Each approach has its own limitations. As for TCR, the observationally constrained results are sensitive to the specification of radiative forcing, which need to be modelled. The ECS also requires knowledge of the heat storage in the Earth system, particularly the oceans. As discussed at length in the second report, our knowledge of the ocean heat content and the processes through which the oceans take up heat is very limited. So, estimating the gain in energy by the current Earth system is very uncertain. This means that observational estimates of the ECS are prone to even greater uncertainty than for the TCR.

The uncertainty in the estimates of ECS from comprehensive climate models stems largely from the incomplete understanding and representation of cloud and other processes. The accuracy of palaeoclimate reconstructions is limited by the uncertainty in the reconstructed temperature, our understanding of the causes of the changes from present climate, and the degree of relevance of past climate change to a warming driven by increases in greenhouse gases.

… Observationally based approaches span a wider range than these model estimates, reflecting the difficulties in estimating radiative forcing and the change in energy content of the Earth system .

… Finally, much of the interest in ECS relates to the amount of warming that would result if the radiative forcing were stabilised. In reality, other Earth system feedbacks, associated for example with the cycling of carbon through natural systems and releases of carbon from permafrost melt, will change, and are likely to increase the actual expected warming (see e.g. Knutti et al 2013).

What is evident is that, even for the more conservative estimates derived from observations, the prospects of substantial global warming by the end of the century are not materially altered by the recent pause in global surface warming.

(9)  “Overestimated global warming over the past 20 years”, John C. Fyfe, Nathan P. Gillett, Francis W. Zwiers, Nature Climate Change, 28 August 2013 — Gated. Abstract:

Recent observed global warming is significantly less than that simulated by climate models. This difference might be explained by some combination of errors in external forcing, model response and internal climate variability.

Excerpt:

Global mean surface temperature over the past 20 years (1993–2012) rose at a rate of 0.14 ± 0.06 °C per decade (95% confidence interval). This rate of warming is significantly slower than that simulated by the climate models participating in Phase 5 of the Coupled Model Intercomparison Project (CMIP5).

… The evidence, therefore, indicates that the current generation of climate models (when run as a group, with the CMIP5 prescribed forcings) do not reproduce the observed global warming over the past 20 years, or the slowdown in global warming over the past fifteen years. [S]uch an inconsistency is only expected to occur by chance once in 500 years, if 20-year periods are considered statistically independent. Similar results apply to trends for 1998–2012. In conclusion, we reject the null hypothesis that the observed and model mean trends are equal at the 10% level.

One possible explanation for the discrepancy is that forced and internal variation might combine differently in observations than in models. … Another possible driver of the difference between observed and simulated global warming is increasing stratospheric aerosol concentrations. Other factors that contribute to the discrepancy could include a missing decrease in stratospheric water vapour, errors in aerosol forcing in the CMIP5 models, a bias in the prescribed solar irradiance trend, the possibility that the transient climate sensitivity of the CMIP5 models could be on average too high or a possible unusual episode of internal climate variability not considered above.

Ultimately the causes of this inconsistency will only be understood after careful comparison of simulated internal climate variability and climate model forcings with observations from the past two decades, and by waiting to see how global temperature responds over the coming decades.

(10)  “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 — Ungated copy here.  From the Georgia Tech press research:

“The stadium wave signal predicts that the current pause in global warming could extend into the 2030s,” said Wyatt …

Curry added, “This prediction is in contrast to the recently released IPCC AR5 Report that projects an imminent resumption of the warming, likely to be in the range of a 0.3 to 0.7 degree Celsius rise in global mean surface temperature from 2016 to 2035.” Curry is the chair of the School of Earth and Atmospheric Sciences at the Georgia Institute of Technology.

The University announcement gives a clear summary. Excerpt:

“The stadium wave signal predicts that the current pause in global warming could extend into the 2030s,” said Wyatt …  Curry added, “This prediction is in contrast to the recently released IPCC AR5 Report that projects an imminent resumption of the warming, likely to be in the range of a 0.3 to 0.7 degree Celsius rise in global mean surface temperature from 2016 to 2035.” Curry is the chair of the School of Earth and Atmospheric Sciences at the Georgia Institute of Technology. … “Current climate models are overly damped and deterministic, focusing on the impacts of external forcing rather than simulating the natural internal variability associated with nonlinear interactions of the coupled atmosphere-ocean system,” Curry said.

(11)  “NAO implicated as a predictor of Northern Hemisphere mean temperature multidecadal variability“, Jianping Li, Geophysical Research Letters, 28 October 2013 — From the abstract:

NHT {Northern Hemisphere mean surface temperature} in 2012–2027 is predicted to fall slightly over the next decades, due to the recent NAO weakening that temporarily offsets the anthropogenically induced warming.

(12)  Global Temperature Update Through 2013, James Hansen, Makiko Sato and Reto Ruedy, 21 January 2014 — Abstract:

The recent slowdown of global warming is a consequence of both a slowdown in the growth rate of climate forcings and recent ENSO history. Given that the tropical Pacific seems to be moving toward the next El Niño, record global temperature is likely in the near term. However, the rate of future warming will depend upon changes of the tropospheric aerosol forcing, which is highly uncertain and unmeasured.

… Assuming that an El Niño begins in summer 2014, 2014 is likely to be warmer than 2013 and perhaps the warmest year in the instrumental record. However, given the lag between El Niño initiation and global temperature, 2015 is likely to have a temperature even higher than in 2014.

(13)  “Recent intensification of wind-driven circulation in the Pacific and the ongoing warming hiatus“, Matthew H. England et al, Nature Climate Change, in press.

Remarks by Professor England in the University of New South Wales press release:

The winds lead to extra ocean heat uptake, which stalled warming of the atmosphere. Accounting for this wind intensification in model projections produces a hiatus in global warming that is in striking agreement with observations. … Unfortunately, however, when the hiatus ends, global warming looks set to be rapid. … This pumping of heat into the ocean is not very deep, however, and once the winds abate, heat is returned rapidly to the atmosphere.

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.

(14)  “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.

Conclusion

The next El Niño, when it occurs in a year or so, may temporarily interrupt the hiatus, but, because the planetary heat sinks in the Atlantic and the Southern Oceans remain intact, the hiatus should continue on a decadal time scale. When the internal variability that is responsible for the current hiatus switches sign, as it inevitably will, another episode of accelerated global warming should ensue.

(15) “Quantifying the likelihood of a continued hiatus in global warming“, Christopher D. Roberts et al, Nature Climate Change, April 2015. Abstract:

Since the end of the twentieth century, global mean surface temperature has not risen as rapidly as predicted by global climate models (GCMs). This discrepancy has become known as the global warming ‘hiatus’ and a variety of mechanisms have been proposed to explain the observed slowdown in warming. Focusing on internally generated variability, we use pre-industrial control simulations from an observationally constrained ensemble of GCMs and a statistical approach to evaluate the expected frequency and characteristics of variability-driven hiatus periods and their likelihood of future continuation. Given an expected forced warming trend of ~0.2 K per decade, our constrained ensemble of GCMs implies that the probability of a variability-driven 10-year hiatus is ~10%, but less than 1% for a 20-year hiatus.

Although the absolute probability of a 20-year hiatus is small, the probability that an existing 15-year hiatus will continue another five years is much higher (up to 25%). Therefore, given the recognized contribution of internal climate variability to the reduced rate of global warming during the past 15 years, we should not be surprised if the current hiatus continues until the end of the decade. Following the termination of a variability-driven hiatus, we also show that there is an increased likelihood of accelerated global warming associated with release of heat from the sub-surface ocean and a reversal of the phase of decadal variability in the Pacific Ocean.

(16)  “Near-term acceleration in the rate of temperature change“, Steven J. Smith, Nature Climate Change, April 2015 — Abstract:

Anthropogenically driven climate changes, which are expected to impact human and natural systems, are often expressed in terms of global-mean temperature. The rate of climate change over multi-decadal scales is also important, with faster rates of change resulting in less time for human and natural systems to adapt. We find that present trends in greenhouse-gas and aerosol emissions are now moving the Earth system into a regime in terms of multi-decadal rates of change that are unprecedented for at least the past 1,000 years. The rate of global-mean temperature increase in the CMIP5 archive over 40-year periods increases to 0.25 ± 0.05 °C (1σ) per decade by 2020, an average greater than peak rates of change during the previous one to two millennia. Regional rates of change in Europe, North America and the Arctic are higher than the global average. Research on the impacts of such near-term rates of change is urgently needed.

(17)  “Ocean impact on decadal Atlantic climate variability revealed by sea-level observations“, Gerard D. McCarthy et al, Nature, 28 May 2015. From the University of Southampton press release:

A new study, by scientists from the University of Southampton and National Oceanography Centre (NOC), implies that the global climate is on the verge of broad-scale change that could last for a number of decades. …

The strength of ocean currents has been measured by a network of sensors, called the RAPID array, which have been collecting data on the flow rate of the Atlantic meridonal overturning circulation (AMOC) for a decade. Dr David Smeed, from the NOC and lead scientist of the RAPID project, adds: “The observations of AMOC from the RAPID array, over the past ten years, show that it is declining. As a result, we expect the AMO is moving to a negative phase, which will result in cooler surface waters. This is consistent with observations of temperature in the North Atlantic.”

Abstract:

Decadal variability is a notable feature of the Atlantic Ocean and the climate of the regions it influences. Prominently, this is manifested in the Atlantic Multidecadal Oscillation (AMO) in sea surface temperatures. Positive (negative) phases of the AMO coincide with warmer (colder) North Atlantic sea surface temperatures. The AMO is linked with decadal climate fluctuations, such as Indian and Sahel rainfall, European summer precipitation, Atlantic hurricanes and variations in global temperatures.

It is widely believed that ocean circulation drives the phase changes of the AMO by controlling ocean heat content. However, there are no direct observations of ocean circulation of sufficient length to support this, leading to questions about whether the AMO is controlled from another source.

Here we provide observational evidence of the widely hypothesized link between ocean circulation and the AMO. We take a new approach, using sea level along the east coast of the United States to estimate ocean circulation on decadal timescales. We show that ocean circulation responds to the first mode of Atlantic atmospheric forcing, the North Atlantic Oscillation, through circulation changes between the subtropical and subpolar gyres — the intergyre region. These circulation changes affect the decadal evolution of North Atlantic heat content and, consequently, the phases of the AMO.

The Atlantic overturning circulation is declining and the AMO is moving to a negative phase. This may offer a brief respite from the persistent rise of global temperatures, but in the coupled system we describe, there are compensating effects. In this case, the negative AMO is associated with a continued acceleration of sea-level rise along the northeast coast of the United States.

(18) “Prospects for a prolonged slowdown in global warming in the early 21st century” by Thomas R. Knutson, Rong Zhang & Larry W. Horowitz, Nature Communications, 30 November 2016 — Abstract:

“Global mean temperature over 1998 to 2015 increased at a slower rate (0.1 K decade−1) compared with the ensemble mean (forced) warming rate projected by Coupled Model Intercomparison Project 5 (CMIP5) models (0.2 K decade−1). Here we investigate the prospects for this slower rate to persist for a decade or more. The slower rate could persist if the transient climate response is overestimated by CMIP5 models by a factor of two, as suggested by recent low-end estimates. Alternatively, using CMIP5 models’ warming rate, the slower rate could still persist due to strong multidecadal internal variability cooling. Combining the CMIP5 ensemble warming rate with internal variability episodes from a single climate model—having the strongest multidecadal variability among CMIP5 models—we estimate that the warming slowdown (

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