The report by the United Nations Intergovermental Panel on Climate Change makes clear that warm weather extremes and heavy precipitation events have increased, most likely as a result of manmade climate change. And it projects with a high degree of confidence increasing hot weather and heavy downpours in the future.
“For the high emissions scenario, it is likely that the frequency of hot days will increase by a factor of 10 in most regions of the world”, said Thomas Stocker, one of the reports co-chairs.
But the “Special Report on Managing the Risks of Extreme Events and Disasters to Advance Climate Change Adaptation” (SREX) is somewhat guarded about the links between manmade climate change and hurricanes, floods, droughts and weather disaster losses. And some of its findings are more conservative and characterized by greater uncertainty than the major volume released by the IPCC in 2007, known as the Fourth Assessment Report (AR4).
So how has the science of climate change and weather extremes changed, and what does it mean?
Perhaps the most watered down conclusion in this report compared to previous assessments relates to changes in tropical storms and hurricanes.
* Whereas AR4 concluded it was likely these storms had increased in intensity in recent decades, SREX finds “there is low confidence that any observed long-term (i.e., 40 years or more) increases in tropical cyclone activity are robust”
* Whereas AR4 concluded it “was more likely than not” manmade climate change played a role in hurricane trends, SREX finds there is “low confidence in attribution of changes in tropical cyclone activity to anthropogenic influences.”
* On future hurricane activity, the SREX agrees with AR4’s projections that peak winds in tropical systems are likely to increase even as the number of these storms stays about the same or decreases.
On the issue of observed and projected changes in floods, SREX comes close to punting:
* It states they can’t tell if floods have increased or decreased: “[we have] overall low confidence at the global scale regarding even the sign of these changes.”
* It states confidence is low in predicting changes in future floods, though based on physical reasoning concludes (with medium confidence) that increases in heavy rainfall should contribute to increases in local flooding
As for droughts, SREX’s conclusions are also highly qualified:
* It states there is medium confidence some parts of the world have experienced more intense and longer droughts while other regions have seen decreases in drought, including central North America. These findings appear to back track a bit from AR4 which stated the global area affected by drought had likely increased.
* SREX is also wishy-washy in its future projections for drought assigning medium confidence that they will intensify in some areas but cautioning elsewhere confidence is low.
Given the record-setting year for billion dollar weather disasters in the U.S., a key question the report tackles is whether this is part of a long-term global trend. The answer is yes, but the report is very cautious in assigning blame to manmade climate change. Instead, it attributes the increase to “increasing exposure of people and economic assets” while stating “a role for climate change has not been excluded.”
The equivocal nature of some of the report’s scientific findings demonstrates the difficulty in drawing conclusions from a patchwork network of weather observations. It also illustrates the challenges in identifying a human fingerprint in data that is inherently noisy due to the highly variable nature of climate and weather, not to mention the challenges in modeling its future behavior.
The report is clear that when it assigns “low confidence” in observed changes, it doesn’t mean the extreme hasn’t changed or won’t change in the future. It simply conveys lack of data and/or knowledge. For some extremes, it seems to be the case that more we learn, the more we recognize we don’t know.
Importantly, the report does not, in any respect, suggest the risks from climate change and mounting greenhouse gas concentrations - to levels not seen in hundreds of thousands of years - have diminished.
NOAA Earth Systems Research Laboratory animation showing CO2 levels rising to levels not seen in at least 800,000 years. (Hat tip Andrew Revkin, dotearth)
If anything, the increased uncertainty about some extremes - both with respect to how they’re changing now and how they might change in the future - might argue that risks are greater, greenhouse gas emissions reductions are more urgent, and more preparedness and societal resilience is required.
Not only is the climate changing, but societal vulnerability is growing as population grows and infrastructure booms. And clearly the risks posed by more severe heat waves and heavy precipitation events - for which there is high confidence - warrant action. This is not to mention the host of risks posed by rising sea levels, ocean acidification, warming effects on plants and animals, etc. - which are climate impacts falling outside the realm of “weather extremes.”
The report makes many prudent recommendations about employing “low regrets” measures for managing climate risks and preparing for weather disasters before they occur while emphasizing the importance of information sharing and learning by doing.
Related: Report: Climate change means more frequent droughts, floods to come (Juliet Eilperin) | NASA scientist Hansen warns “climate dice” already loaded for more extreme weather (Andrew Freedman) | When should we blame climate change for natural disasters? (Wonkblog, Brad Plumer)