6 November 2015

Did Climate Change Make “This” Storm Worse? Perhaps.

Posted by Dan Satterfield

The AMS has published its annual look at how climate change may have impacted certain major weather events, and it makes for some fascinating reading. This kind of research is called an attribution study, and they can be very informative. One of the most famous, was done by Dr. Ben Santer (Nat. Academies) that showed how different greenhouse warming looks vs. warming due to an increase in energy from the sun. It destroyed the often heard climate denier canard that the sun is responsible for the warming planet (see below).

Notice how the warming we have seen in the atmosphere tracks very closely to what you would expect from rising greenhouse gases and is opposite to that expected if it were the sun.

If the sun were responsible and not greenhouse gases, the stratosphere would not be cooling. If greenhouse gases were the main factor, we’d see warming low and cooling in the stratosphere. This is exactly what we are seeing.

The attribution studies released today, in the Bulletin of The American Meteorological Society (BAMS), are worth a read, and the abstract is here:

ABSTRACT—Stephanie C. Herring, Martin P. Hoerling, James P. Kossin, Thomas C. Peterson, and Peter A. Stott

Screen Shot 2015-11-05 at 9.21.36 PMUnderstanding how long-term global change affects the intensity and likelihood of extreme weather events is a frontier science challenge. This fourth edition of explaining extreme events of the previous year (2014) from a climate perspective is the most extensive yet with 33 different research groups exploring the causes of 29 different events that occurred in 2014. A number of this year’s studies indicate that human-caused climate change greatly increased the likelihood and intensity for extreme heat waves in 2014 over various regions. For other types of extreme events, such as droughts, heavy rains, and winter storms, a climate change influence was found in some instances and not in others. This year’s report also included many different types of extreme events. The tropical cyclones that impacted Hawaii were made more likely due to human-caused climate change. Climate change also decreased the Antarctic sea ice extent in 2014 and increased the strength and likelihood of high sea surface temperatures in both the Atlantic and Pacific Oceans. For western U.S. wildfires, no link to the individual events in 2014 could be detected, but the overall probability of western U.S. wildfires has increased due to human impacts on the climate.

Challenges that attribution assessments face include the often limited observational record and inability of models to reproduce some extreme events well. In general, when attribution assessments fail to find anthropogenic signals this alone does not prove anthropogenic climate change did not influence the event. The failure to find a human fingerprint could be due to insufficient data or poor models and not the absence of anthropogenic effects. This year researchers also considered other human caused drivers of extreme events beyond the usual radiative drivers. For example, flooding in the Canadian prairies was found to be more likely because of human land-use changes that affect drainage mechanisms. Similarly, the Jakarta floods may have been compounded by land-use change via urban development and associated land subsidence. These types of mechanical factors reemphasize the various pathways beyond climate change by which human activity can increase regional risk of extreme events.

One thing to remember is that in a very real sense, ALL storms are climate related. The planet is warmer than it was, and the ocean temperatures globally are at levels never measured before. Warmer air means more water vapor in the atmosphere, and thus we see heavier rain and snow events, and this has been measured (see below).