Kevin E Trenberth; 7 November 2016
The role of human-induced climate change on damaging climate extremes
Kevin E. Trenberth
National Center for Atmospheric Research (NCAR), P.O. Box 3000, Boulder, CO 80307, U.S.A.
Phone: (303) 497 1318; Fax: (303) 497 1333
email: trenbert@ucar.edu
3 November 2016
Summary
Weather and climate extremes happen all of the time, even in an unchanging climate. Yet there is a
justifiably strong sense that some of these extremes are becoming more frequent, and that the main
reason is human-induced climate change. Indeed, the main way in which climate change is likely to
affect societies around the world is through changes in extremes.
How global warming affects extreme events
Changes in atmospheric composition from human activities, primarily the burning of fossil fuels and
deforestation, enhance the greenhouse effect, although with important regional effects from aerosol
particulates. Globally on a day-to-day basis these effects are 1 to 2% of the flow of natural energy
through the climate system. However, because global warming is always heating the planet, there is a
much bigger impact from the cumulative effects. Hence, all weather events are now occurring in an
environment which has changed in significant ways.
The main memory of these changes is through the warming of the oceans and the loss of Arctic sea ice.
Sea surface temperatures have warmed by more than 1°F since the 1970s, and over the oceans this has
led to 4 to 6% more water vapor in the atmosphere. The warmer and moister environment in turn has
likely led to a 5 to 10% effect on storms that exceeds previous bounds for extreme weather events.
Consequently, global warming has a direct influence on changes in precipitation and heavy rains, as well
as contributing to increased temperatures and heat waves. Increased heating leads to greater
evaporation and thus surface drying, thereby increasing intensity and duration of drought events.
However, the water-holding capacity of air increases by about 4% per 1°F warming (or 7% per 1°C) ,
which leads to increased water vapor in the atmosphere, and this provides the biggest influence on
precipitation. Storms, whether individual thunderstorms, extratropical rain or snow storms, or tropical
cyclones and hurricanes, supplied by increased moisture, produce more intense precipitation events,
even in places where total precipitation is decreasing. In turn this increases the risk of flooding. At the
same time, dry spells in between such events also increase.
There have also been observed changes in where it rains, with dry areas becoming drier (generally
throughout the subtropics) and wet areas becoming wetter, especially in mid to high latitudes. This
pattern is what we would expect from models that simulate global warming and is projected to continue
into the future.
Conventional attribution
There is a tremendous desire to attribute causes to weather and climate events that is often challenging
from a physical standpoint. The climate community has responded to the demand for timely information
by attempting to perform attribution of climate extremes, partly through the IPCC reports but also
through annual reports offering closer to real-time assessments and posing the question of whether the
likelihood or strength of the event was affected by anthropogenic climate change.