News•September 26, 2013

5 Things to Watch for in Friday’s IPCC Climate Report

On Friday morning, the U.N. Intergovernmental Panel on Climate Change will release the first part of its latest study on the state of climate science. The report, known as the Fifth Assessment Report, will form the scientific foundation for governments to consider taking action to reduce global warming emissions, including serving as a centerpiece for negotiations on a new global climate accord that are set to take place in 2015.

IPCC Working Group I chapter authors listen as scientists and government delegates discussed the wording of the Summary for Policymakers at a meeting in Stockholm on Sept. 25, 2013.
Credit: IISD.

Right now in Stockholm, scientists and government representatives (the “intergovernmental” part of the IPCC’s name) are in intense and, by all accounts, excruciatingly slow negotiations over the precise wording of the most widely-read portion of the new report, known as the Summary for Policymakers (SPM).

Drafts of the report have been circulating for several weeks, and while there is little groundbreaking in the report — after all, it synthesizes previously published work, rather than conducting new research — there are some key areas where the Fifth Assessment Report departs from the previous report that came out in 2007.

There are also key issues to monitor to see if representatives change the draft language to reflect their special interests.

Here are five important areas to look at on Friday morning:

1. How does the panel explain the slowdown in the rate of global warming since about 1998?

This is a hot-button issue in Stockholm as government negotiators have pressed scientists to refine their draft language on the recent warming slowdown to better indicate that some computer models failed to predict this development, and to more clearly explain why the short-term warming slowdown is consistent with the long-term warming of the climate system.

According to the draft, the report states that the rate of warming over the past 15 years is about 0.09°F, which is smaller than the trend since 1951, which is about 0.21°F.

The draft does note that each of the past three decades has been warmer than all preceding decades since 1850, and the period from 1983-2012 was “likely” the warmest 30-year period of the past 1,400 years.

The draft faults some climate models for failing to accurately simulate the shorter-term warming slowdown, suggesting that this might be because of “unpredictable” occurrences that can influence the climate on short timescales, such as volcanic eruptions and solar cycles.

Another section of the draft again pins the reduction in warming from 1998-2012 on internal climate variability, such as fluctuations in ocean temperatures, as well as the cooling influence of volcanic eruptions and an inactive solar cycle. However, the report draft said it is difficult to determine the relative role played by each possible factor.

The warming slowdown language is likely to be adjusted, at least slightly, in the final version.

2. How does the report express levels of scientific certainty, compared to the 2007 report?

The draft contains several different ways of expressing how confident scientists are in the various conclusions of the report. For example, the report uses the estimates that previous reports had used, with “virtually certain” corresponding to a 99-100 percent probability, and “very likely” to a 90-100 percent probability.

The past three decades have been the warmest since global instrument records began in the late 1800s.
Click image to enlarge.

But the draft report also includes summary terms to describe the available evidence, such as “limited” or “robust,” and for the degree of agreement among the evidence, such as “very low or very high.”

Also, the report uses levels of confidence to express particular conclusions, such as “medium confidence.”

Much of this uncertainty language is new and is intended to respond to critics of previous reports. However, the revisions may end up making things more confusing, given the combination of probabilities, degree of agreement expressions, and confidence levels, often in the same paragraph.

3. How different are the sea level rise projections compared to the 2007 report?

The 2007 report projected a global sea level rise of .7.1 to 23.2 inches by 2100, but it did not include the influence of rapid melting of the Greenland ice sheet as well as portions of Antarctica because not enough information was known at the time.

Now, with more data, the new report is expected to revise its sea level rise projections upward, although still to a range that is below what several smaller studies have projected.

The draft shows that the global average sea level is projected to rise by an average of 16 to 24 inches by 2100, depending on future greenhouse gas emissions and the sensitivity of the climate system to higher amounts of greenhouse gases.

“There’s a much bigger improvement in the discussion of sea level rise than there was before,” Gavin Schmidt, a NASA climate scientist, said in an interview.

Schmidt said with sea level rise, there are still considerable uncertainties about the stability of polar ice sheets, which is something that the report’s authors have been grappling with. “How do you deal with situations where there are these big unknowns? The IPCC has done a better job with that this time (than in 2007),” he said.

4. Has the estimate of the climate’s sensitivity to a doubling of carbon dioxide concentrations changed significantly since 2007?

Like previous IPCC reports, the draft contains a range for the “equilibrium climate sensitivity,” which is essentially an estimate of how much warming would occur if the amount of CO2 in the atmosphere were to double.

Scientists have been making such estimates since the late 19th century, and throughout the IPCC years (the organization was created in 1988), the figures have stayed within a narrow range.

With the Fifth Assessment Report, the climate sensitivity range may be adjusted slightly lower, which is something that many climate skeptics have already seized upon to argue that global warming won’t be as significant as previously thought.

The draft shows a likely climate sensitivity range of 2.7°F to 8.1°F, down from the 2007 IPCC report that pegged it at a “likely” range of 3.6°F to 8.1°F.

Climate scientists said the slight change is insignificant, and in no way indicates that climate change is likely to be less severe than previously projected.

Andrew Dessler, a climate researcher at Texas A&M University, said that the climate sensitivity adjustment is one of the few areas of disagreement between the new report and 2007’s Fourth Assessment Report, but that it is not a significant difference.

“The key thing to note is that the main conclusions are identical to every previous IPCC report. Here’s the proof: the change that has everyone in a tizzy is a slight enlargement of one side of the error bar for climate sensitivity,” Dessler said in an email. “If that’s the biggest change, then things are not changing very much.”

And Noah Diffenbaugh, a climate scientist at Stanford University, told Climate Central in an interview that the biggest source of uncertainty in projecting future global warming isn’t the climate sensitivity, but rather “the human dimension,” or in other words, what man-made emissions will be.

The climate sensitivity is not a prediction of how much global warming is expected to occur, considering that right now the world is on course to greatly exceed a doubling of CO2, without enacting strict emissions cuts. It does, however, play an important role in calculating future warming rates.

5. What will the report say about future temperature projections based on revised future emissions scenarios?

The report will project future global average temperature changes based on revised scenarios of future greenhouse gas concentrations compared to the scenarios used in the 2007 report, which were calculated using different criteria. The draft states that the global mean surface temperature change for the period 2016 to 2035 will “likely” be in the range of 0.54°F to 1.26°F above average temperatures from 1986-2005, depending on global greenhouse gas emissions, and the sensitivity of the climate system to those emissions.

By the end of the century, global average temperatures will likely range from 4.7°F to 8.64°F above 1986-2005 levels, again depending on global greenhouse gas emissions and the sensitivity of the climate system.

For three out of the five emissions scenarios used in the new report, global average temperatures are likely to exceed 2.7°F above preindustrial levels by the end of the century, and for two of the scenarios with higher emissions, they are expected to exceed 3.6°F above preindustrial levels by the end of the century.

At the U.N. climate summit in Copenhagen in 2009, global leaders committed to containing global warming to under 3.6°F (or 2°C) above pre-industrial levels. Political leaders determined that if warming were to exceed 3.6°F, the risk of dangerous consequences, from sea level rise to extensive melting of polar ice sheets, would escalate significantly.