Groundhogs, tornadoes, hurricanes, and the new IPCC report

Punxsutawney Phil, the fearless groundhog forecaster of Punxsutawney, Pennsylvania, did not see his shadow this morning, signifying that winter will come to an early close. Phil can expect this to be a common occurrence in the coming millennia, according to the newly released Summary of Policy Makers issued today by the United Nations sponsored Intergovernmental Panel on Climate Change (IPCC). In their fourth report since 1990, the IPCC offers its strongest language yet that Earth's climate is warming and humans are largely responsible:

"Warming of the climate system is unequivocal, as is now evident from observations of increases in global average air and ocean temperatures, widespread melting of snow and ice, and rising global mean sea level."

"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. Discernible human influences now extend to other aspects of climate, including ocean warming, continental-average temperatures, temperature extremes, and wind patterns."

Earth has warmed, sea levels have begun to rise at an accelerated rate, and Northern Hemisphere snow cover has decreased substantially over the past 150 years (Figure 1). These facts are not controversial. The big change from the IPCC's last report, in 2001, is the level of confidence on if humans are to blame. In that report, human-emitted (anthropogenic) greenhouse gases were estimated to be likely responsible for Earth's temperature increase (67-90% chance), while the new report says it is very likely (greater than 90% chance).


Figure 1. Observed changes in (a) global average surface temperature; (b) global average sea level rise from tide gauge (blue) and satellite (red) data and (c) Northern Hemisphere snow cover for March-April. All changes are relative to corresponding averages for the period 1961-1990. Smoothed curves represent decadal averaged values while circles show yearly values. The shaded areas are the uncertainty intervals estimated from a comprehensive analysis of known uncertainties (a and b) and from the time series (c). Image credit: FIGURE SPM-3 from the Summary of Policy Makers from the 2007 Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report.

Predicted temperature rise
The 2007 IPCC report predicts temperature rises of 1.1 - 6.4°C (2 - 11.5°F) by 2100. This is a wider range than the 1.4 - 5.8°C increase given in the 2001 report. However, the 2007 report goes on to say that their best estimate for temperature rise is 1.8 - 4°C (3.2 - 7.1°F).

Predicted sea level rise
The 2007 report predicts that sea level rise by 2100 will be .6 - 1.9 feet (18 - 58 cm). An additional 3.9 to 7.8 inches (10 to 20 cm) are possible if the recent surprising melting of polar ice sheets continues. The 2001 IPCC report gave a much wider range for sea level rise: .3 - 2.9 feet (8 - 88 cm).

What does the IPCC say about stronger hurricanes?
The IPCC did a good job with their treatment of how global warming is affecting hurricanes. Here's their carefully worded statement on the observed changes:

There is observational evidence for an increase of intense tropical cyclone activity in the North Atlantic since about 1970, correlated with increases of tropical sea surface temperatures. There are also suggestions of increased intense tropical cyclone activity in some other regions where concerns over data quality are greater. Multi-decadal variability and the quality of the tropical cyclone records prior to routine satellite observations in about 1970 complicate the detection of long-term trends in tropical cyclone activity. There is no clear trend in the annual numbers of tropical cyclones.

Later in the report, there is a table that shows that there has likely (>66% chance) been an increase in strong hurricanes since 1970 in some regions. It isn't mentioned, but the Atlantic is the region where this increase has been most notable. Also in that table is the assertion that it is more likely than not (>50% chance) that there has been a human contribution to this trend. This statement was leaked to the press yesterday, and resulted in speculation that the IPCC concluded that stronger hurricanes like Katrina were due to human-caused global warming. However, there is a footnote on the table, which wasn't part of yesterday's leaked press reports: "Magnitude of anthropogenic contributions not assessed. Attribution for these phenomena based on expert judgment rather than formal attribution studies."

In other words, the link between stronger hurricanes and global warming is a theory (expert judgment) and is not a conclusion of the IPCC. It is reasonable to theorize that some human contribution is responsible for the increase in strong hurricanes in the Atlantic since 1970, since this increase does correlate so well with the observed increase in sea surface temperatures. However, as mentioned in the earlier paragraph, it is difficult to make a strong statement saying that global warming is responsible for stronger hurricanes, due to the high natural variability of these storms and the poor observational record: "multi-decadal variability and the quality of the tropical cyclone records prior to routine satellite observations in about 1970 complicate the detection of long-term trends in tropical cyclone activity." The IPCC table is confusing, and I believe it was a mistake to assign a probability of how likely a human contribution to hurricane intensity has been. There is not enough good science to make a sound judgment, and this section of the table should have been left blank.

Finally, the IPCC projection for how climate change will affect hurricanes in the future is pretty non-controversial, since they don't attach any numbers saying how large these effects will be:

Based on a range of models, it is likely that future tropical cyclones (typhoons and hurricanes) will become more intense, with larger peak wind speeds and more heavy precipitation associated with ongoing increases of tropical SSTs. There is less confidence in projections of a global decrease in numbers of tropical cyclones. The apparent increase in the proportion of very intense storms since 1970 in some regions is much larger than simulated by current models for that period.

Overall, the IPCC statements on hurricanes are very similar to those adopted by the World Meteorological Organization (WMO) in December, as I discussed in an earlier blog. The WMO report concluded, "Though there is evidence both for and against the existence of a detectable anthropogenic signal in the tropical cyclone climate record to date, no firm conclusion can be made on this point."

What does the IPCC say about stronger tornadoes?
In the wake of today's devastating tornadoes that swept through Central Florida, killing at least 19 people, it is important to ask how climate change might be affecting tornadoes. Well, we don't have good enough long-term observational records of tornadoes to tell, and climate models don't shed any light on the issue, either. Here's the relevant statement in the 2007 IPCC report:

There is insufficient evidence to determine whether trends exist in small scale phenomena such as tornadoes, hail, lighting, and dust storms.

I'll have much more on the IPCC report next week, as will Dr. Ricky Rood of the University of Michigan, who will be writing in our new featured Climate Change blog. Who knows, if we want to collect a quick $10,000, we can "thoughtfully explore the limitations of climate model outputs", as explained in an article posted by the UK Guardian today.


Jeff Masters