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

Summary:  What can we expect from Earth’s climate during the next decade or so?  Unfortunately climate sciences forecasts have become footballs in public policy debates.  Can we see through the fog?  Here we see a selection of articles explaining the current state of the climate sciences.  This is the fourth in a series of posts; at the end are links to the other chapters.

Update:  On advice from a reader I’ve split this post into two shorter posts.  The second part is up here.

Contents

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.

1. An introduction to the debate
2. How long until we have a clear trend in global temperatures?
3. Other chapters in this series
4. For more information

(1)  An introduction to the debate

To understand future climate change we must understand the causes of the current stabilization (brief or long as it may be).  A recommended introduction to the issues:  “Provoked scientists try to explain lag in global warming“, Environment & Energy Publishing, 25 October 2011 — This is a comprehensive review, well-worth reading in full.  Excerpt:

By any measure, the decade from 2000 to 2010 was the warmest in modern history. However, 1998 remains the single warmest year on record, though by some accounts last year tied its heat. Temperatures following 1998 stayed relatively flat for 10 years, with the heat in 2008 about equaling temperatures at the decade’s start. The warming, as scientists say, went on “hiatus.”

The hiatus was not unexpected. Variability in the climate can suppress rising temperatures temporarily, though before this decade scientists were uncertain how long such pauses could last. In any case, one decade is not long enough to say anything about human effects on climate; as one forthcoming paper {Santer et al, see below} lays out, 17 years is required.

… “It has always bothered me,” said Kevin Trenberth, head of the climate analysis section at the National Center for Atmospheric Research. “Natural variability is not a cause. One has to say what aspect of natural variability.” Trenberth’s search has focused on what he calls “missing energy,” which can be thought of as missing heat. The heat arriving and leaving the planet can be measured, if crudely, creating a “budget” of the Earth’s energy. While this budget is typically imbalanced — the cause of global warming — scientists could account for where the heat was going: into warming oceans or air, or melting ice. In effect, the stall in temperatures meant that climatologists no longer knew where the heat was going. It was missing.

… “What’s really been exciting to me about this last 10-year period is that it has made people think about decadal variability much more carefully than they probably have before,” said Susan Solomon, an atmospheric chemist and former lead author of the United Nations’ climate change report, during a recent visit to MIT. “And that’s all good. There is no silver bullet. In this case, it’s four pieces or five pieces of silver buckshot.”

This buckshot has included some familiar suspects, like the Pacific’s oscillation between La Niña and El Niño, along with a host of smaller influences, such as midsize volcanic eruptions once thought unable to cool the climate. The sun’s cycles are proving more important than expected. And there are suspicions that the vast uptick in Chinese coal pollution has played a role in reflecting sunlight back into space, much as U.S. and European pollution did decades ago.

… Indeed, the most important outcome from the energy hunt may be that researchers are chronically underestimating air pollution’s reflective effect, said NASA’s James Hansen, head of the Goddard Institute for Space Studies. Recent data has forced him to revise his views on how much of the sun’s energy is stored in the oceans, committing the planet to warming. Instead, he says, air pollution from fossil fuel burning, directly and indirectly, has been masking greenhouse warming more than anyone knew.

(2)  How long until we have a clear trend in global temperatures?

“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. Abstract (red emphasis added):

We compare global-scale changes in satellite estimates of the temperature of the lower troposphere (TLT) with model simulations of forced and unforced TLT changes. While previous work has focused on a single period of record, we select analysis timescales ranging from 10 to 32 years, and then compare all possible observed TLT trends on each timescale with corresponding multi-model distributions of forced and unforced trends. We use observed estimates of the signal component of TLT changes and model estimates of climate noise to calculate timescale-dependent signal-to-noise ratios (S/N). These ratios are small (less than 1) on the 10-year timescale, increasing to more than 3.9 for 32-year trends. This large change in S/N is primarily due to a decrease in the amplitude of internally generated variability with increasing trend length.

Because of the pronounced effect of interannual noise on decadal trends, a multi-model ensemble of anthropogenically-forced simulations displays many 10-year periods with little warming. A single decade of observational TLT data is therefore inadequate for identifying a slowly evolving anthropogenic warming signal. Our results show that temperature records of at least 17 years in length are required for identifying human effects on global-mean tropospheric temperature.

(3)  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

(4)  For More Information

The sociology and politics of climate science:

1. A look at the science and politics of global warming, 12 June 2008
2. President Kennedy speaks to us about global warming and Climate Science, 7 August 2008
3. “Aliens cause global warming”: wise words from the late Michael Crichton, 15 November 2008
4. My “wish list” for the climate sciences in 2009, 2 January 2009
5. Peer review of scientific work – another example of a flawed basis for public policy, 22 January 2009
6. Richard Feynmann, one of the 20th centuries greatest scientists, talks to us about climate science, 12 February 2009