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IPCC SPM – So What’s new?

February 2nd, 2007 by Sean Davis · No Comments

Much of the information that is in the IPCC Summary for Policymakers is not news to those of us who are in the atmospheric science community and follow the literature. Even though we try, things do tend to slip through the cracks, and it takes someone (or 2500+ people, to be precise) to distill the progress in the field over the past years into a manageable piece of information.

As a first cut at digesting the 4th Assessment Report (AR4), below I’ve tried to summarize those things I noticed that are different from the Third Assessment Report (TAR). As one would expect, since TAR there have been more improvements in the measurements, monitoring, modelling, and validation of the information that goes into the IPCC WG1 assessment. As a result, in many cases confidence levels have changed regarding the state of the science. And, of course, there are things that are new and hot topics. As a comparison, below I’ve put the TAR and AR4 radiative forcing summary plots, which are related to some of the points below


TAR Radiative Forcings


AR4 Forcing

Here are some of those things that jumped out at me on reading this report:

  • Since TAR, aerosol radiative forcings are better understood due to improved measurements, “but they remain the dominant uncertainty in radiative forcing” (see figure above)
  • Changes in solar irradiance are now estimated at 0.12 W m-2, which is “less than half the estimate given in TAR
  • Regarding low- and mid-tropospheric warming rates,

New analyses of balloon-borne and satellite measurements of lower- and mid-tropospheric temperature
show warming rates that are similar to those of the surface temperature record and are consistent within
their respective uncertainties, largely reconciling a discrepancy noted in the TAR

  • Regarding the Greenland and Antarctic ice sheets,

New data since the TAR now show that losses from the ice sheets of Greenland and Antarctica have very
likely contributed to sea level rise over 1993 to 2003

  • New observational evidence for an increase in tropical cyclone activity. Although this is a highly contentious subject, I thought they did a good job presenting the state of the science and what is not known. They basically say that there has been no observed trend in the overall number of TC’s, and that models project an increase in the 21st century. They also recognize that there are still serious discrepancies between models and the observed increase in TC activity.
  • They note that in TAR a decrease in diurnal temperature range was reported, whereas the updated data set they used in this report showed no changes (TAR data was 1950-1993, AR4 data for this analysis was 1979-2004)
  • This one is the media attention grabber — Note that in the IPCC context likely and very likely have specific meanings (>66% confidence, and >90% confidence, respectively)

Most of the observed increase in globally averaged temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations. This is an advance since the TAR’s conclusion that “most of the observed warming over the last 50 years is likely to have been due to the increase in greenhouse gas concentrations. (emphasis mine)

  • Since TAR, Coupled climate models have been shown to accurately simulate the observed temperature changes on each of the continents, providing “stronger evidence of human influence on climate than was available in the TAR
  • AR4 contains the first climate sensitivity constraint (1 sigma confidence interval is 2-4.5 C for a doubling of CO2), whereas the TAR simply reported the range of climate sensitivites (as I recall… I don’t have that in front of me right now). Part of this is due to the observational constraints provided by things like the Mt. Pinatubo eruption and the 20th century data record.
  • Also — and this is good news for my job security –

Cloud feedbacks remain the largest source of uncertainty.

  • As for projections of future climate, the AR4 boasts a “large number of simulations available from a broader range of models” than the TAR. They don’t give specifics here, but I’m sure that will be listed in the full report.
  • For each of the future emissions scenarios, however, the best estimate for the temperature change over the next century is within 10% of the TAR values. So no drastic changes on the future temperature predictions. Just a narrowing of the range.
  • Since TAR, studies of the projected changes in precipitation patterns indicate that increases in precipitation are very likely (>90% confidence) for high latitudes.
  • I couldn’t tell if this is new or not, but they note that the multi-model mean slowdown in the Atlantic Meridional Overturning Circulation (MOC) is 25%. I didn’t realize it was that high! And I don’t recall TAR having any predictions of that sort. …But then, I’m no oceanographer. Maybe this is old news.

Whew… Well, there’s much more to say about the IPCC SPM, but I thought it was important to start off by straightnening out in my head what the new and different information is in the AR4 SPM compared to TAR. If I have the energy, I’ll try to get around to talking about some of the other interesting, surprising, or new-to-me points brought up in the SPM next week.

Tags: climate · general interest · modeling

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