Regarding whether climate change is a factor in catastrophic natural disasters of this year... like the flood in Pakistan... and the heat wave in Russia and drought in Colorado, which each resulted in major wildfires... Hansen writes:
In other words... "if the question were posed as "would these events have occurred if atmospheric carbon dioxide had remained at its pre-industrial level of 280 ppm?", an appropriate answer in that case is "almost certainly not." That answer, to the public, translates as "yes", i.e., humans probably bear a responsibility for the extreme event."
Read the complete analysis by Dr. Hansen below... and watch out for 2012!
Dr. James Hansen - How Warm Was This Summer?
Let's look at the surface temperatures in the summer of 2010, which justifiably received a
lot of attention. Figure 1 shows maps of the June-July-August temperature anomaly (relative to
1951-1980) in the GISS temperature analysis (described in paper in press at Rev. Geophys.,
available http://data.giss.nasa.gov/gistemp/paper/gistemp2010_draft0803.pdf) for 2009 and
2010, as well as maps for December-January-February (Northern Hemisphere winter, Southern
Hemisphere summer) for the past two years.
June-July-August 2010 was the 4th warmest in the 131 year GISS analysis, while 2009
was the 2nd warmest(1). 2010 was a bit cooler than 2009 mainly because a moderate El Nino in the
equatorial Pacific Ocean during late 2009 and early 2010 has been replaced by a moderate La
Nina. Also most of Antarctica was cool in winter 2010, while it was warm in 2009. Antarctic
winter temperature anomalies are very noisy, fluctuating chaotically from year to year.
The maps make clear that perceptions of how hot it was depend on where you live. The
two warmest anomalies on the planet this past summer were Eastern Europe and the Antarctic
Peninsula. Not many people live on the Antarctic Peninsula and an anomaly of even several
degrees in winter there is not a big deal. But the warm anomaly centered in Eastern Europe,
which covered most of Europe and the Middle East, was noticed, to say the least. It was also
quite warm in Japan, where the prior summer had been cooler than the 1951-1980 mean. The
United States, which had been unusually cool in the summer of 2009, was warm this past
summer, except the Pacific Northwest, which was cooler than the 1951-1980 climatology.
(1) Exact rankings differ slightly among the analyses made by different groups, primarily because of different
approaches for estimating temperature anomalies in data sparse regions, as discussed in Rev. Geophys. preprint.
Figure 1. Seasonal-mean temperature anomalies relative to 1951-1980 mean for the
most recent two summers and winters.
Figure 2. Winter and summer temperature anomalies over United States, Europe
and Japan relative to 1951-1980 mean. Areas employed to calculate anomalies were the 48
contiguous states for the United States, rectangle defined by 36-70N latitude and 10W-30E
longitude for Europe, and 40 1-by-1 degree boxes approximately covering Japan. If the
box defining Europe were extended to the east to encompass western Russia, the 2010
anomaly would be comparable to the warm anomaly in 2003.
These global temperature anomaly maps may help people understand that the temperature
anomaly in one place in one season has limited relevance to global trends. Unfortunately it is
common for the public to take the most recent local seasonal temperature anomaly as indicative
of long-term climate trends. Last winter in the Northern Hemisphere (left side of Figure 1)
provided a good example of this misperception. As discussed in the Rev. Geophys. paper, the
extreme winter cold anomalies in Eurasia and the United States were a fluke associated with the
most extreme Arctic Oscillation in the record.
This does not mean that local anomalies are unrelated to global trends, but it is necessary
to look at statistics. Figure 2 shows winter and summer surface temperature anomalies averaged
over the United States (contiguous 48 states), Europe and Japan. In each of these locations either
7 or 8 of the last 10 winters were warmer than the 1951-1980 mean winter temperature. Summer
temperatures are a bit less noisy: 8 of the last 10 summers were warmer than the 1951-1980
mean in the United States and Japan, and 10 of 10 in Europe. So if you are perceptive and old
enough, you should be able to notice a trend toward warmer seasons.
Extreme anomalies get the most attention, and rightly so because they have the greatest
practical impact. Figure 2 is relevant to the likelihood of having extreme climate anomalies. For
example, the curve for European summer temperatures shows how the baseline is shifting. The
hot summer of 2003 was so far above the long-term mean summer temperature that it may have
seemed to be a once in 1000 years fluke. However, Figure 2 shows that the baseline for summer
temperature in Europe has changed as global warming occurred over the past few decades. That
trend is expected to continue if greenhouse gases continue to increase, so it will not be surprising
if an extremely warm summer anomaly occurs there again within the next several years.
Figure 3. Seasonal temperature anomalies relative to 1951-1980 mean for the globe
and for low latitudes. Nino 3.4 index is as defined in Rev. Geophys. preprint.
Figure 3 has graphs of the global and low latitude seasonal temperature anomalies. The
low latitude anomalies are strongly dependent on the El Nino-La Nina cycle of equatorial Pacific
Ocean temperature anomalies, as shown by the Nino 3.4 SST2. Global temperature anomalies
tend to reflect Nino variability, with, on average, a lag of about three months.
The global seasonal temperature anomaly for March-April-May in 2010 was the warmest
in the 131 year GISS temperature data set. The low latitude temperature anomaly was less than
in 1998, as the recent El Nino was much weaker than the one in 1998. The June-July-August
temperature anomalies dropped as the equatorial Pacific Ocean has moved into the La Nina
phase. Computer models suggest that the La Nina may peak near the end of 2010. Regardless of
how long the current La Nina extends, the next two or three seasonal-mean global and low
latitude temperature anomalies are likely to be cooler than the anomalies for the past four
seasons.
The figures here are updates of or closely related to figures in the Rev. Geophys. paper. The definition of Nino
index that we use and other details can be found in that paper.
Figure 4. January-August surface temperature anomalies during three specific
years in the GISS analysis, and comparison of global monthly anomalies for those years.
Figure 4 provides an indication of the likely effect of the current cooling trend on the
rank of the 2010 calendar year temperature anomaly. The maps compare January-August
temperature anomalies for 2010, 2005 (the warmest year in the GISS analysis), and 1998 (one of
the warmest years in the GISS analysis, the temperature being boosted by the "El Nino of the
century"). 2010 is clearly the warmest of these years for the first eight months. However, the 4th
section of Figure 4 shows that the monthly anomalies in 2010 have declined steadily over the
past five months as the Pacific Ocean moved into the La Nina phase. The last four months of
2005 (green line in Figure 4) were unusually warm, so it is not possible to say yet whether 2005
or 2010 will be the warmest calendar year in the GISS analysis. It is likely that the 2005 and
2010 calendar year means will turn out to be sufficiently close that it will be difficult to say
which year was warmer, and results of our analysis may differ from those of other groups. What
is clear, though, is that the warmest 12-month period in the GISS analysis was reached in mid-
2010, as shown in the Rev. Geophys. preprint.
Projections of trends over the next few years are possible based on the following
considerations: (1) the planet is out of energy balance by at least several tenths of one W/m2 due
to the rapid increase of greenhouse gases during the past few decades, as confirmed by
measurements of changing ocean heat content, (2) inertia of energy systems that assures
continuing growth of atmospheric CO2 by about 2 ppm per year for the next few years, (3)
expectation that the solar irradiance will climb out of the recent long-lasting solar minimum, as
shown in Figure 5, (4) model projections suggesting that the current La Nina may bottom out
near the end of 2010. Given the dominant effect of El Nino-La Nina on short-term temperature
change and the usual lag of a few months between the Nino index and its effect on global
temperature, it is unlikely that 2011 will reach a new global record temperature.
Figure 5. Solar irradiance through June 2010. [Fröhlich & Lean, Astron. Astrophys.
Rev. 12, 273, 2004. http://www.pmodwrc.ch/pmod.php?topic=tsi/composite/SolarConstant]
In contrast, it is likely that 2012 will reach a record high global temperature. The
principal caveat is that the duration of the current La Nina could stretch an extra year, as some
prior La Ninas have (see Nino 3.4 index at the bottom of Figure 3). Given the association of
extreme weather and climate events with rising global temperature, the expectation of new
record high temperatures in 2012 also suggests that the frequency and magnitude of extreme
events could reach a high level in 2012. Extreme events include not only high temperatures, but
also indirect effects of a warming atmosphere including the impact of higher temperature on
extreme rainfall and droughts. The greater water vapor content of a warmer atmosphere allows
larger rainfall anomalies and provides the fuel for stronger storms driven by latent heat.
Finally, a comment on frequently asked questions of the sort: Was global warming the
cause of the 2010 heat wave in Moscow, the 2003 heat wave in Europe, the all-time record high
temperatures reached in many Asian nations in 2010, the incredible Pakistan flood in 2010? The
standard scientist answer is "you cannot blame a specific weather/climate event on global
warming." That answer, to the public, translates as "no".
However, if the question were posed as "would these events have occurred if atmospheric
carbon dioxide had remained at its pre-industrial level of 280 ppm?", an appropriate answer in
that case is "almost certainly not." That answer, to the public, translates as "yes", i.e., humans
probably bear a responsibility for the extreme event.
In either case, the scientist usually goes on to say something about probabilities and how
those are changing because of global warming. But the extended discussion, to much of the
public, is chatter. The initial answer is all important.
Although either answer can be defended as "correct", we suggest that leading with the
standard caveat "you cannot blame..." is misleading and allows a misinterpretation about the
danger of increasing extreme events. Extreme events, by definition, are on the tail of the
probability distribution. Events in the tail of the distribution are the ones that change most in
frequency of occurrence as the distribution shifts due to global warming.
For example, the "hundred year flood" was once something that you had better be aware
of, but it was not very likely soon and you could get reasonably priced insurance. But the
probability distribution function does not need to shift very far for the 100-year event to be
occurring several times a century, along with a good chance of at least one 500-year event.
Printer-friendly version
