Kevin Trenberth: Human influence on extreme climate events is greater than conventional analyses suggest

News Release Summary for Trenberth et al. Nature Climate Change paper.

Human-caused climate change is making many weather and climate extremes more frequent and/or intense.

The conventional approach to extreme event attribution tends to underestimate the human influence. A new paper “Attribution of climate extreme events” by Kevin Trenberth and colleagues in Nature Climate Change suggests that addressing this problem lies, in part, in separating atmospheric dynamics from thermodynamics.

Atmospheric dynamics deals with the phenomena and their development and movement; they are short-lived and inherently unpredictable. Indeed, extreme weather events are by their nature always unique. On the other hand, thermodynamics involves changes in temperature and temperature extremes, and the associated changes in atmospheric moisture vapor that lead to more intense rains and flood risks, as well as droughts in which the extra heat from increased greenhouse gases accumulates. For these larger thermodynamic influences, more reliable and useful statements can be made about the role of human-caused climate change in extreme events.

Thus, a more robust and societally relevant approach to extreme event attribution is to look at how the impacts of particular events were affected by observed changes in the environment where they develop: the air and sea surface temperature, sea level, and atmospheric moisture content.

The paper notes that sea-surface temperatures have increased by about 0.6 °C or 1 °F globally since the 1950s due to human-caused warming. Atmospheric moisture associated with this increase has risen by about 5%. Global sea level has risen by about 19 cm or 7.5 inches, with regional variations. These factors play a key role in feeding moisture into storms, intensifying them, causing heavy rains, and amplifying storm surges. Among the several examples discussed in the paper are these two:

In October 2012’s Superstorm Sandy, for example, high sea-surface temperatures were responsible for greatly increasing the storm’s strength (as measured by storm depth and wind speed) and for increasing precipitation by 35%. Even if just a third or a half of the increased sea-surface temperature is attributable to human-caused warming, then the storm surge and associated damage was considerably influenced by climate change. Indeed, the subways and tunnels might not have flooded without the warming-induced increases in sea level and storm intensity and size, putting the price tag of human-caused warming on this storm in the tens of billion of dollars.

In the Boulder floods of September 2013, extremely high sea-surface temperatures off the west coast of Mexico, and the associated record atmospheric water vapor amounts that flowed into Colorado as a result, were instrumental in the event, and it probably would not have occurred without human-caused warming. Such an increase in atmospheric water vapor becomes concentrated when focused by topography, as it did in Boulder, and further amplified on the ground as water drains into channels and rivers. This suggests an important role for human-caused warming in those Boulder floods.

In summary, the paper emphasizes that the climate is changing. We have a new normal. The environment in which all weather events occur is not what it used to be: all storms, without exception, are different. Even if they look like ones we used to have, they are not the same. The most useful way to evaluate the effects of human-caused warming on extreme events is to focus on the influences of the changed large-scale thermodynamic environment on the extremes of temperatures and moisture associated with the event.