Friday :: Feb 2, 2007


by Christina Hulbe

The Intergovernmental Panel on Climate Change (IPCC) issued the first volume of its Fourth Assessment Report (AR4), The Physical Science Basis, early this morning. It represents the combined efforts of hundreds of scientists whose primary ambition is to get the science as right as they can and make the best future projections possible. It also reflects, to some degree, the efforts of politicians, whose motives may be more complicated (the US delegation to the final ratifying meeting is led by political appointees and the word is that they are very finicky).

The AR4 provides an overview of change since the start of the 20th Century and model-based projections of future change. Here a provide a few quick highlights. The major points of the report can be read in the Summary for Policy Makers pdf.

The full report is long and will require time to read and absorb. I'm familiar with a lot of the science but far from all of it. In the following I provide a few quick highlights along with some useful links.

what's happening now
The consensus report highlights the anthropogenic (human origin) component of the global warming signal as well as the future warming to which we have already committed ourselves by past fossil fuel burning. We discussed that, and the sea level rise associated with it, in a recent post.

The connection between anthropogenic greenhouse gas emissions and global warming was established preliminarily in the Third Assessment Report (TAR). It was shown there that models driven by natural variability only and anthropogenic forcings only could not reproduce observed global temperature trends from 1850 to 2000 but that models including both could. The AR4 supports that conclusion more firmly than did the TAR.

Global warming is resulting in both increased precipitation and heightened drought, depending on region. We should expect increased precipitation in some regions (eastern North & South America, northern Europe, and northern & central Asia) because a warmer atmosphere can hold (and release) more water vapor than can a drier atmosphere. Along with that comes an increase in the frequency of intense precipitation events. We should expect drying in some regions (the Sahel, Mediterranean, southern Africa, and parts of southern Asia) as a warmer atmosphere drives more evaporation.

The Arctic is warming at nearly twice the rate of the rest of the planet. The Arctic Climate Impact Assessment issued in November of 2004 is a great resource on this topic. The Synthesis Report provides a thorough, easy to read overview (with nice graphics).

Widespread changes in extreme temperatures have been observed. Globally, the number of nights with frost is decreasing while "heat waves" have become more frequent.

The warming of the last half century is demonstrated to be anomalous in at least the previous 1300 years. The last time the Arctic was significantly warmer than at present was the last interglacial, 125,000 years ago. It is worth noting that at that time (the last interglacial), sea level was 4 to 6 meters higher than at present. Such dramatic rises are not projected in the AR4 but the ice is there to do it (as discussed in the recent post on sea level rise).

the future
The "best estimate" global mean warming projection over the 21st Century is 3 degrees Celsius (5.4 Fahrenheit) over the pre-industrial value. If you want a point for comparison, a recent coupled climate model estimate for the total global cooling between full glacial and pre-industrial modern conditions is about 4.5 degrees Celcius (pdf of a technical report).

The future projections in the AR4 cover a range of possibilities, from relatively modest warming and sea level rise to dramatic events that would cause significant hardship for people across the planet. Those end members are both attractive to different groups for different reasons. What we need to keep in mind as readers of the news stories, editorials, and the AR4 itself is that the spread is primarily a result of uncertainty in future socio-economic development, technological progress, and decisions about fossil fuel burning. To deal with those uncertainties, a panel on emissions scenarios produced a standard set of storylines, each with its own set of scenarios that embrace a range of possible futures. This is good news, in a way. We can, at least to some extent, decide which storyline we follow and what future is ours. That's what endeavors like the 1997 Kyoto Protocol meeting are supposed to be about.

All of the climate modeling centers participating in the IPCC AR4 effort use the same set of storylines and scenarios so that differences arising from different attributes of the various climate models can be detected and also assessed. Inter-model comparison is an important pathway through which we can determine what is robust in the models and what is not.

Uncertainty analysis is an essential part of the IPCC AR4 process. It begins with agreed upon methods by which uncertainty may be evaluated. A fundamental goal is to provide data with which planners and policy makers (and others) can engage in meaningful risk analysis.

It takes a lot of work by a host of dedicated scientists to produce a report like the AR4. It synthesizes the work of many more. The workshops and expert meetings at which people working in many specialty areas met to discuss, debate, and refine what would go into the AR4 began as early as 2003. You can get a feel for just what goes into this by scanning the last two year's IPCC calendar here. And hey, if you know a scientist who works in a climate-related field, give 'em a pat on the back today. This process and this report reflect us all.

Christina Hulbe :: 6:35 AM :: Comments (21) :: Digg It!