You're Getting Warmer

What is the IPCC, and what's the deal with its new report?

When climate change emerged as an important environmental issue in the late 1980s, the world governments' first response was to establish an international body to produce summaries of scientific knowledge of climate change. That body is the Intergovernmental Panel on Climate Change. The IPCC has completed three major reports since its formation, in 1990, 1995, and 2001, and throughout 2007 will release its Fourth Assessment Report (hereafter referred to as the AR4).

The AR4 has been eagerly awaited because, in the six years since the 2001 report was published, our knowledge of the climate system has dramatically improved. As a result, the 2001 report has gotten quite out-of-date and no longer represents our most up-to-date view of the climate problem.

On Friday, Feb. 2, the IPCC released the first part of the AR4. Friday's release was the Summary for Policymakers (SPM) [PDF] for the working group that covers the basic science of climate change (other reports on impacts, adaptation, and mitigation will be published later this year). The SPM is a short summary of the science written in plain language for those without scientific training. The rest of the AR4 report will be periodically released throughout 2007.

The IPCC reports are widely regarded as the authoritative statements of scientific knowledge about climate change, and as such they carry enormous weight in both the scientific and policy communities. The immense credibility of the IPCC's reports arises from the credible process that produces it. The reports are based on the peer-reviewed literature and are written by hundreds of expert climate scientists from over 100 countries. The reports then go through multiple layers of review, including expert peer review by thousands of climate scientists who were not authors of the report.

The IPCC's Third Assessment Report, published in 2001, then went through review by a blue-ribbon panel convened by the U.S. National Academy of Sciences, which endorsed its findings. The conclusions of the IPCC reports have also been endorsed by the American Geophysical Union, the American Meteorological Society, the American Association for the Advancement of Science, and others.

The resulting IPCC reports are accepted worldwide as the best summaries of what the scientific community knows about climate change and how confidently we know it.

The Highlights

My overall first impression was pleasant surprise at the strong wording in the document. Assessments, like the science that underlies them, tend to be conservative, so strong statements are often couched in so many caveats that they come out with the consistency of soggy toast. The statements in the AR4 SPM are crisp and clear and tough, reflecting the fact that our knowledge of the climate system is now so good that few caveats are necessary.

Here are some important new results from today's summary:

Confidence intervals
Before I begin with the highlights, I have to talk about confidence levels and how they are integrated in the IPCC reports. All important statements in the IPCC report have levels of confidence associated with them. After all, some things we know with essentially 100 percent confidence (e.g., the observed increase in CO2 is caused by human activities), while others are much more uncertain (e.g., there is an increase in hurricane strength over the past several decades associated with human activities). In the IPCC document, confidence is expressed using a carefully defined set of terms:

In this Summary for Policymakers, the following terms have been used to indicate the assessed likelihood, using expert judgment, of an outcome or a result:

Virtually certain > 99% probability of occurrence
Extremely likely > 95%
Very likely > 90%
Likely > 66%
More likely than not > 50%
Unlikely < 33%
Very unlikely < 10%
Extremely unlikely < 5%.

This allows the readers to assess the strength of the various claims being made.

Are humans causing climate change?
Over time, the IPCC's statements about the contribution of humans to our present-day warming have become much stronger.

1990: "The size of this warming is broadly consistent with prediction of climate models, but it is also of the same magnitude as natural climate variability. Thus the observed increase could be largely due to this natural variability"

1995: "the balance of evidence suggests a discernible human influence on the climate"

2001: "most of the observed warming over the last 50 years is likely to have been due to the increase in greenhouse gas concentrations"

And now ... drum roll, please ... 2007: "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 2007 statement increases our certainty that humans are the dominant influence on the climate from "likely" (66%) to "very likely" (90%). The statement continues the trend of the IPCC to make ever-stronger statements --- a result of ever-stronger underlying science.

Projections of warming over the next century
In the 2001 report, the IPCC gave a single range for the increase in global and annual average temperature of 1.4 to 5.8° Celsius. This range was difficult to interpret because it was presented without any associated probability (e.g., Was it a 95% confidence interval, or 99%? Was probability evenly distributed within the range, or peaked toward the center?)

The AR4 SPM provides a more nuanced view of the projections of future warming. In particular, "best estimates" as well as "likely ranges" are provided for each scenario. The best estimates for the six scenarios range from 1.8 to 4.0°C for 21st century warming. Including the "likely range," temperature increases are projected to range from 1.1 to 6.4°C.

From a policy standpoint, the changes on warming projections between the 2001 report and today's report are insignificant. In both reports, the upper end of the range is high enough that catastrophic impacts for everyone on the planet could result. The low end of the range would present manageable impacts for rich countries like the U.S. For poor countries, even the low end of the range would likely present insurmountable impacts.

The AR4 SPM also says that, if we stopped emitting greenhouse gases today, the Earth would still warm by 0.6°C during the 21st century -- as much as the temperatures warmed during the 20th century. That's a sobering realization.

Also, the AR4 SPM finally revises our estimates of "climate sensitivity" (the equilibrium temperature increase after a doubling of carbon dioxide). For the last 30 years, it has been 1.5 to 4.5°C. The AR4 SPM says:

It is likely to be in the range 2 to 4.5°C with a best estimate of about 3°C, and is very unlikely to be less than 1.5°C. Values substantially higher than 4.5°C cannot be excluded, but agreement of models with observations is not as good for those values.

Sea-level rise
Increases in sea level are one of the most certain consequences of global warming, and one of the most frightening. In the IPCC's 2001 report, sea-level rise during the 21st century was estimated to be 9 to 88 cm. In the AR4 SPM, the increase in sea level falls in the range of 18 to 59 cm. This reduced range reflects great improvement in our knowledge of the factors that control sea level.

It should be noted that the 18 to 59 cm estimate is really a lower limit:

The projections include a contribution due to increased ice flow from Greenland and Antarctica at the rates observed for 1993-2003, but these flow rates could increase or decrease in the future. For example, if this contribution were to grow linearly with global average temperature change, the upper ranges of sea level rise for SRES [Special Report on Emission Scenarios] scenarios ... would increase by 0.1 m to 0.2 m. Larger values cannot be excluded, but understanding of these effects is too limited to assess their likelihood or provide a best estimate or an upper bound for sea level rise.

Aerosol forcing
Aerosols are tiny particles, either solid or liquid, suspended in the atmosphere. Fuel combustion and other human activities release them to the atmosphere, which can either warm or cool the Earth's surface depending on their composition. Black carbon aerosols (tiny particles of soot) absorb both sunlight and upwelling infrared radiation, and so warm the surface. Liquid sulfate aerosols reflect sunlight back to space, and so cool the surface.

In the 2001 report, aerosols were one of the most uncertain aspects of climate change. However, the AR4 SPM makes a strong case that our knowledge of aerosols has dramatically improved, arguing that aerosols' dominant effect is to cool the climate:

Anthropogenic contributions to aerosols (primarily sulphate, organic carbon, black carbon, nitrate and dust) together produce a cooling effect, with a total direct radiative forcing of -0.5 [-0.9 to -0.1] W m-2 and an indirect cloud albedo forcing of -0.7 [-1.8 to -0.3] W m-2. These forcings are now better understood than at the time of the [2001 report] due to improved in situ, satellite and ground-based measurements and more comprehensive modelling, but remain the dominant uncertainty in radiative forcing. Aerosols also influence cloud lifetime and precipitation.

The hockey stick
Over the past few years, everyone's favorite time waster was the hockey-stick debate.

The AR4 SPM weighs in on this argument:

"Paleoclimate information supports the interpretation that the warmth of the last half century is unusual in at least the previous 1300 years."

and

Average Northern Hemisphere temperatures during the second half of the 20th century were very likely higher than during any other 50-year period in the last 500 years and likely the highest in at least the past 1,300 years. Some recent studies indicate greater variability in Northern Hemisphere temperatures than suggested in the [2001 report], particularly finding that cooler periods existed in the 12 to 14th, 17th, and 19th centuries.

This statement basically validates the 2001 report's original statement about the hockey stick. I'm somewhat surprised at this no-holds-barred endorsement, particularly since a National Academy of Science panel that reviewed the science of the hockey stick in 2006 explicitly did not make such a strong endorsement.

The AR4 SPM does, however, endorse the strong statement made by the Academy panel that it is very likely that the Earth has been warming for the last 500 years.

Conclusions
Over the past five years, there have been virtually no breakthrough findings that revolutionized the science of climate change. There have been some tremendous scientific results, but they have largely confirmed and refined what we already thought we knew: the climate is warming, humans are playing a role, and we can expect further warming of a few degrees if we don't reduce our emissions of greenhouse gases.

The stability of the dominant climate-science paradigm should be both reassuring and unsettling: reassuring because it suggests we understand the climate pretty well; unsettling because it forecasts potentially serious impacts if we don't take action soon. There's a tremendous amount of information in the AR4 SPM. Read the report for yourself [PDF] --- and then write your representatives in Congress.