New York Times - Justin Gillis
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| The aftermath of a bush fire in Victoria, Australia, in 2015, which scientists reported was the hottest year in the historical record. Credit David Crosling/European Pressphoto Agency |
Scientists reported Wednesday that 2015 was the
hottest year in the historical record by
far, breaking a record set only the year before — a burst of heat that
has continued into the new year and is roiling weather patterns all over
the world.
In the contiguous United States, the year was
the second-warmest on record, punctuated by a December that was both the hottest and the wettest since record-keeping began. One result has been a wave of
unusual winter floods coursing down the Mississippi River watershed.
Scientists
started predicting a global temperature record months ago, in part
because an El Niño weather pattern, one of the largest in a century, is
releasing an immense amount of heat from the Pacific Ocean into the
atmosphere. But the bulk of the record-setting heat, they say, is a
consequence of the long-term planetary warming caused by human emissions
of greenhouse gases.
"The
whole system is warming up, relentlessly," said Gerald A. Meehl, a
scientist at the National Center for Atmospheric Research in Boulder,
Colo.
It
will take a few more years to know for certain, but the back-to-back
records of 2014 and 2015 may have put the world back onto a trajectory
of rapid
global warming, after a period of relatively slow warming dating to the last powerful El Niño, in 1998.
Politicians
attempting to claim that greenhouse gases are not a problem seized on
that slow period to argue that "global warming stopped in 1998," with
these claims and similar statements reappearing recently on the
Republican presidential campaign trail.
Statistical
analysis suggested all along that the claims were false, and that the
slowdown was, at most, a minor blip in an inexorable trend, perhaps
caused by a temporary increase in the absorption of heat by the Pacific
Ocean.
"Is
there any evidence for a pause in the long-term global warming rate?"
said Gavin A. Schmidt, head of NASA's climate-science unit, the Goddard
Institute for Space Studies, in Manhattan. "The answer is no. That was
true before last year, but it's much more obvious now."
The Hottest Year on Record
Globally, 2015 was the warmest year in recorded history.
How far above or below average temperatures were in 2015
Compared with the average from 1901 to 2000
Michael
E. Mann, a climate scientist at Pennsylvania State University,
calculated that if the global climate were not warming, the odds of
setting two back-to-back record years would be remote, about one chance
in every 1,500 pairs of years. Given the reality that the planet is
warming, the odds become far higher, about one chance in 10, according
to Dr. Mann's calculations.
Two
American government agencies — NASA, the National Aeronautics and Space
Administration, and NOAA, the National Oceanic and Atmospheric
Administration — compile separate analyses of the global temperature,
based upon thousands of measurements from weather stations, ships and
ocean buoys scattered around the world. Meteorological agencies in
Britain and Japan do so, as well. The agencies follow slightly different
methods to cope with problems in the data, but obtain similar results.
The
American agencies released figures on Wednesday showing that 2015 was
the warmest year in a global record that began, in their data, in 1880.
British scientists released figures showing 2015 as the warmest in a
record dating to 1850. The Japan Meteorological Agency had already
released preliminary results showing 2015 as the warmest year in a
record beginning in 1891.
On
Jan. 7, NOAA reported that 2015 was the second-warmest year for the
lower 48 United States. That land mass covers less than 2 percent of the
surface of the Earth, so it is not unusual to have a slight divergence
between United States temperatures and those of the planet as a whole.
The
end of the year was especially remarkable in the United States, with
virtually every state east of the Mississippi River having a record warm
December, often accompanied by heavy rains.
A
warmer atmosphere can hold more water vapor, and an intensification of
rainstorms was one of the fundamental predictions made by climate
scientists decades ago as a consequence of human emissions. That
prediction has come to pass, with the rains growing more intense across
every region of the United States, but especially so in the East.
The
term global warming is generally taken to refer to the temperature
trend at the surface of the planet, and those are the figures reported
by the agencies on Wednesday.
Some
additional measurements, of shorter duration, are available for the
ocean depths and the atmosphere above the surface, both generally
showing an inexorable long-term warming trend.
Most
satellite measurements of the lower and middle layers of the atmosphere
show 2015 to have been the third- or fourth-warmest year in a 37-year
record, and scientists said it was slightly surprising that the huge El
Niño had not produced a greater warming there. They added that this
could yet happen in 2016.
When
temperatures are averaged at a global scale, the differences between
years are usually measured in fractions of a degree. In the NOAA data
set, 2015 was 0.29 degrees Fahrenheit warmer than 2014, the largest jump
ever over a previous record. NASA calculated a slightly smaller figure,
but still described it as an unusual one-year increase.
The
intense warmth of 2015 contributed to a heat wave in India last spring
that turns out to have been the second-worst in that country's history,
killing an estimated 2,500 people. The long-term global warming trend
has exacted a severe toll from extreme heat, with eight of the world's
10 deadliest heat waves occurring since 1997.
Only
rough estimates of heat deaths are available, but according to figures
from the Center for Research on the Epidemiology of Disasters, in
Brussels, the toll over the past two decades is approaching 140,000
people, with most of those deaths occurring during a European heat wave
in 2003 and a Russian heat wave in 2010.
The
strong El Niño has continued into 2016, raising the possibility that
this year will, yet again, set a global temperature record. The El Niño
pattern is also disturbing the circulation of the atmosphere,
contributing to worldwide weather extremes that include a drought in
southern Africa, threatening the food supply of millions.
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