What can climate scientists tell us about future weather? Part two of two.

Summary:  This is a review of the climate science literature, giving a perspective different than that shown us by the news media.  This is just a selection from the vast, rapidly growing body of research on the frontiers of knowledge about this vital subject.  As past comments to thei series show, true believers on both sides of the climate wars react to real science like vampires to holy water.  This is the fifth in a series of posts; at the end are links to the other chapters.

Contents

These posts show a tiny sliver of the wide range of work conducted today in the climate sciences.  This gives a look at what’s happening on the cutting edge of research, so poorly reported by the general news media.

1. Looking at the big picture:  drivers of the two-centuries-long warming
2. Causes of the recent pause in warming
3. We want good forecasts. Can we have them? Can scientists forecast climate trends?
4. Other chapters in this series
5. For more information

(1)  Looking at the big picture:  drivers of the two-centuries-long warming

To forecast what is to come we must understand what produced the past trend — warming over the past 2 centuries.

(a)  A clear summary of the situation, although written by an amateur:  “Understanding the Global Warming Debate“, Warren Meyer, Forbes, 9 February 2012

(b)  “Patterns of change: whose fingerprint is seen in global warming“, Gabriele Hegerl, Francis Zwiers and Claudia Tebaldi, Environmental Research Letters, 7 December  2011 — Excerpt:

Rigorous quantitative analyses of the patterns of change in the temperature of the atmosphere and ocean observed over the past half-century, incorporating all known uncertainties in the observations, in our knowledge of climate variability, and feedbacks, underpin the assessment that most of the warming of the past 50 years is ‘very likely’ (more than 90% likelihood) due to anthropogenic increases in greenhouse gases.

(c)  “Trends, change points & hypotheses“, Judith Curry posted at Climate Etc, 7 February 2012 — Excerpt:

Consider the following three hypotheses that explain 20th century climate variability and change, with implied future projections:

I.  IPCC AGW hypothesis: 20th century climate variability/change is explained by external forcing, with natural internal variability providing high frequency ‘noise’.

In the latter half of the 20th century, this external forcing has been dominated by anthropogenic gases and aerosols. The implications for temperature change in the 21st century is 0.2C per decade until 2050. Challenges: convincing explanations of the warming 1910-1940, explaining the flat trend between mid 1940′s and mid 1970′s, explaining the flat trend for the past 15 years.

II.  Multi-decadal oscillations plus trend hypothesis: 20th century climate variability/change is explained by the large multidecadal oscillations (e.g NAO, PDO, AMO) with a superimposed trend of external forcing (AGW warming).

The implications for temperature change in the 21st century is relatively constant temperatures for the next several decades, or possible cooling associated with solar. Challenges: separating forced from unforced changes in the observed time series, lack of predictability of the multidecadal oscillations.

III:  Climate shifts hypothesis: 20th century climate variability/change is explained by synchronized chaos arising from nonlinear oscillations of the coupled ocean/atmosphere system plus external forcing (e.g. Tsonis, Douglass).

The most recent shift occurred 2001/2002, characterized by flattening temperatures and more frequent LaNina’s. The implications for the next several decades are that the current trend will continue until the next climate shift, at some unknown point in the future. External forcing (AGW, solar) will have more or less impact on trends depending on the regime, but how external forcing materializes in terms of surface temperature in the context of spatiotemporal chaos is not known. Note: hypothesis III is consistent with Sneyers’ arguments re change-point analysis. Challenges: figuring out the timing (and characteristics) of the next climate shift.

There are other hypotheses, but these three seem to cover most of the territory. The three hypotheses are not independent, but emphasize to varying degrees natural internal variability vs external forcing, and an interpretation of natural variability that is oscillatory versus phase locked shifts. Hypothesis I derives from the 1D energy balance, thermodynamic view of the climate system, whereas Hypothesis III derives from a nonlinear dynamical system characterized by spatiotemporal chaos. Hypothesis II derives from climate diagnostics and data analysis.

Each of these three hypotheses provides a different interpretation of the 20th century attribution and has different implications for 21st century climate. Hypothesis III is the hypothesis that I find most convincing, from a theoretical perspective and in terms of explaining historical observations, although this kind of perspective of the climate system is in its infancy.

(2) Causes of the recent pause in warming

(a)  About the power hitter of greenhouse gases

While the news media chats away about CO2 CO2 CO2, climate scientists watch a wider range of factors.  Such as water vapor, the power hitter among Earth’s greenhouse gases.  CO2 warms by increasing the amount of water vapor, starting positive feedback which current models say will rapidly heat the planet.  So it’s watched:  “Contributions of Stratospheric Water Vapor to Decadal Changes in the Rate of Global Warming“, Susan Solomon et al, Science, 5 March 2010 — Free 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.

(b)  Significance of the recent pause in warming

Most scientists consider climate trends significant only over periods of 15+ years.  One major study says that 17+ years are necessary:  “Separating signal and noise in atmospheric temperature changes: The importance of timescale“, Ben Santer et al, Geophysical Research Letters, 18 November 2011 — Free copy here.

(c)  Whatever its statistical significance, the causes of the pause in warming remain unclear

“Reconciling anthropogenic climate change with observed temperature 1998–2008“, Robert K. Kaufmann et at, Proceedings of the National Academy of Science, 19 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.

(3)  We want good forecasts.  Can we have them? Can scientists forecast climate trends?

We don’t have sufficient data yet for an answer.  Here are some scientists grappling with this important question.

(a) The literature has frequent mentions — cautionary notes, not conclusions — like this one

“Climate Chaotic Instability: Statistical Determination and Theoretical Background“, Raymond Sneyers, Evnironmetrics, September/October 1997:

Of course, it should not be discounted that in longer developments this chaotic component would appear to be linked to a longer range regularity. However, the results found for the considered climato- logical series, suggesting that the perturbation of any regularity by chaotic components is an intrinsic property for each particular level of time or space scales, mean accepting the fact that the evolution of such series remains practically out of control, making validation of any model statistical or stochastic, practically impossible.

(b) But there is a powerful example of what might be a successful prediction

“Advancing decadal-scale climate prediction in the north atlantic sector“, N. S. Keenlyside1 et al, Nature, May 2008 — Free copy here. Slides to their presentation are here. Four years later their forecast has proven correct, so far.

Abstract (red emphasis added):

The climate of the North Atlantic region exhibits fluctuations on decadal timescales that have large societal consequences. Prominent examples include hurricane activity in the Atlantic1, and surface-temperature and rainfall variations over North America, Europe and northern Africa. Although these multidecadal variations are potentially predictable if the current state of the ocean is known, the lack of subsurface ocean observations that constrain this state has been a limiting factor for realizing the full skill potential of such predictions. Here we apply a simple approach— that uses only sea surface temperature (SST) observations — to partly overcome this difficulty and perform retrospective decadal predictions with a climate model. … Thus these results point towards the possibility of routine decadal climate predictions.

Using this method, and by considering both internal natural climate variations and projected future anthropogenic forcing, we make the following forecast: over the next decade, the current Atlantic meridional overturning circulation will weaken to its long-term mean; moreover, North Atlantic SST and European and North American surface temperatures will cool slightly, whereas tropical Pacific SST will remain almost unchanged. 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.

(c)  Here’s one scientist’s answer

“Climate science: Decadal predictions in demand“, Mark A. Cane, Nature Geoscience, 21 March 2010 — Free copy here.

Over the past decade, the mean global temperature did not rise much, if at all. This pause in global warming cannot be attributed to cutbacks in greenhouse-gas emissions by the planet’s human population, so it must be nature taking a turn towards colder temperatures. The extent to which such natural climate variability can be predicted on decadal timescales is not known.

Creating useful climate predictions is not straightforward. … The Miami workshop turned the attention of both climate scientists and stakeholders to decadal predictions of climate, a field that is still in its infancy (if not prenatal). Decadal climate predictions aim to cover the gap between seasonal to interannual prediction with lead times of two years or less and projections of climate change a century ahead.

… Therefore, the hope for useful skill in predicting natural variability is far from assured. The climate system is chaotic and it is not known how predictable decadal variations are, even if we had perfect models and sufficient observations to determine the initial state with high precision.

Perhaps there is something special about a decadal timescale that affords predictability, but perhaps not: decadal variability may just be the part of a featureless spectrum of frequencies that we happen to pick out of our instrumental records because they are only about 150 years long. Several valuable efforts to extend the record with palaeoproxy data were reported (L. Stott, Univ. Southern California, USA; B. Horton, Univ. Pennsylvania, USA; P. Swart and H. Wanless, Univ. Miami, USA), but these data are still too sparse and uncertain to allow a firm assessment.

As one example, according to the current leading idea, the Atlantic Multidecadal Oscillation is generated by atmospheric noise (E. Schneider, Center for Ocean, Land, Atmosphere/George Mason Univ., USA). If indeed random processes are at the heart of decadal variability, as strongly supported by the talk of C. Deser (National Center for Atmospheric Research, USA), this might seem to doom hopes for decadal predictability. However, the possibility remains that, once underway, the evolution of important patterns of variability could be projected forward.

… Demand for accurate decadal prediction is running ahead of supply. This imbalance challenges us to determine how much of the natural climate variability that will contribute substantially to global climate in the next few decades is predictable. Even if it turns out that climate is essentially not predictable at a decadal scale, projections of future climate will be misleading unless we gain a fuller understanding of the range of natural variability in store for us in a warmer world.

(4)  Chapters in this series

1. What we know about our past climate, and its causes
2. Good news!  Global temperatures have stabilized, at least for now.
3. Is it possible to debate climate change with true believers? See the replies to Thursday’s post.  Comments welcomed!
4. What can climate scientists tell about the drivers of future warming?
5. What can climate scientists tell us about the drivers of future warming?  – part two of two
6. The slow solar cycle is getting a lot of attention. What are its effect on us?
7. What we’re learning about climate, and recommendations

(5)  For More Information

Past forecasts:

1. Lies told under the influence of the Green religion to save the world, 30 July 2010
2. A new video about global warming, a Leftists’ wet dream pretending to be humor, 1 October 2010
3. More about the forecast for flooded cities in the late 21st century, 16 October 2010
4. Looking into the past for guidance about warnings of future climate apocalypses, 17 October 2010

Cautionary notes:

1. An example of important climate change research hidden, lest it spoil the media’s narrative, 22 May 2009
2. An important step to take before we spend a trillion dollars to save the planet from global warming, 31 January 2010
3. Quote of the day – hidden history for people who rely on the mainstream media for information, 12 February 2010
4. The hidden history of the global warming crusade, 19 February 2010
5. “Most scientific papers are probably wrong” – New Scientist, 20 June 2010
6. “Climate Change: what do we know about the IPCC?”, 27 June 2010
7. Damn the research! We need to act now to stop global warming., 17 August 2010
8. Puncturing the false picture of a scientific consensus about the causes and effects of global warming, 20 September 2010