The document summarizes Martin P. Hoerling's response to criticisms of claims made in a New York Times Op-Ed by James E. Hansen regarding the impacts of climate change. Hoerling takes issue with several specific assertions made by Hansen, arguing they are contrary to peer-reviewed literature and climate change assessments. He provides analysis and references to studies to support his counterarguments. The overall summary is that the certainty expressed in Hansen's claims is not supported by the current scientific understanding of regional climate change projections and uncertainties.

1. Martin P. Hoerling, a federal research meteorologist specializing in climate dynamics,
has written the following expansion and defense of his criticism of some assertions
made in an Op-Ed article on climate change by James E. Hansen of NASA. His initial
criticism was posted on the Dot Earth blog.
Several scientific conclusions and claims were expressed in the NYT Op-Ed piece by
J. Hansenwith which I raised concerns and objections on a scientific basis. While
allowing for imprecision in how nuanced climate change science is sometimes
communicated in such venues, there were nonetheless statements in the Hansen
NYT piece that drew my attention because they stood contrary to peer-reviewed
literature. Some of these claims could also be tested and/or falsified by simple tests
using data available in the public domain, with examples given below.
The Hansen NYT piece asserts:
"Over the next several decades, the Western United States and the semi-arid region
from North Dakota to Texas will develop semi-permanent drought, with rain, when
it does come, occurring in extreme events with heavy flooding. Economic losses
would be incalculable. More and more of the Midwest would be a dust bowl.
California’s Central Valley could no longer be irrigated. Food prices would rise to
unprecedented levels."
To which I replied:
“I am unaware of any projection for "semi-permanent" drought in this time frame
over the expansive region of the Central Great Plains. He implies the drought is to
be a phenomena due to lack of rain (except for the brief, and ineffective
downpours). I am unaware of indications, from model projections, for a material
decline in mean rainfall. “
Supporting Material for My Statement
The peer-reviewed study by Milly et al. titled “Global Pattern of Trends in
Streamflow and Water Availability in a Changing Climate”(Nature, 2005) used the
IPCC CMIP3simulations to diagnose the relative change in surface runoff for the
period 2040-2060 compared to 1900-1970. A version of their published Fig. 4 is
shown below. Runoff is a holistic indicator of surface water balance that integrates
the effects of changes in mean precipitation and its characteristics, and also changes
in temperature via evapotranspiration. These published results indicate no
appreciable change in surface water availability over the semi-arid region from
North Dakota to Texas, or for the Midwest as a whole, within this ensemble of
CMIP3 simulations. The authors point out the various uncertainties in such
regional scale projections, not the least of which one must also include the
uncertainty in the consequences of changes in land cover and land use.

2. The recently published report by the U.S. Global Change Research Program titled
“Global Climate Change Impacts in the United States”provided a synthesis of the
current understanding of probable regional climate change impacts. This synthesis
document states, in the section on Water Resources on pg. 45, “Precipitation and
runoff are likely to increase in the Northeast and Midwest in winter and spring.”
Drawn from such works as the Milly et al. study, this synthesis is clearly quite
contrary to the assertion made in the Hansen NYT piece.Regarding flooding that is
mentioned in the Hansen piece, it is important not to confuse heavy downpours
with hydrologic flooding on a river basin scale, for instance. The IPCC Special
Report on Managing the Risks of Extreme Events and Disasters to Advance Climate
Change Adaptation (SREX, 2012http://www.ipcc-wg2.gov/SREX/) states a low
confidence for both observed and projected changes in the magnitude and
frequency of floods.
Summary
The claim in the Hansen NYT piece that the Midwest would be a dustbowl in coming
decades thus runs contrary to peer reviewed literature and recent assessments by
the U.S. Global Research Program that emerged from the synthesis of current
understanding by an expert team of scientists.
Figure 1.The relative change in runoff in the twenty-first century expressed as the ensemble (arithmetic)
mean of relative change (percentage) in runoff for the period 2041–60, computed as 100 times the
difference between 2041–60 runoff in the SRESA1B experiments and 1900–70 runoff in the 20C3M
experiments, divided by 1900–70 runoff. Based on Fig. 4 from Milly et al. (2005). [Milly, P, K. Dunne, A.
Vecchia, Nature, 438, 2005, doi:10.1038/nature04312]. Left-side illustrates runoff change for drainage
basin scale, and right side for geopolitical state scales.

3. Regarding observed changes in climate of the Great Plains, I stated:
“Indeed, that region (Great Plains) has seen a general increase in rainfall over the
long term, during most seasons (certainly no material decline). Also, for the warm
season when evaporative loss is especially effective, the climate of the central Great
Plains has not become materially warmer (perhaps even cooled) since 1900. In
other words, climate conditions in the growing season of the Central Great Plains
are today not materially different from those existing 100 years ago. This
observational fact belies the expectations, from climate simulations, and in truth,
our science lacks a good explanation for this discrepancy. “
Supporting Material for My Statement
The lack of a warming trend over the central United States during the past century,
sometimes called the U.S. “warming hole”, has been especially noted in the peer-
reviewed literature (e.g., Kunkel et al. Journal of Climate, 2006, Knutson et al. Journal of
Climate2006). Particularly striking has been a cooling trend in summertime temperatures
at many meteorological observing stations located between the Rocky and Appalachian
Mountains over the period 1901-2010 (Fig. 2, top). This region of summertime cooling
has generally coincided with a region of summertime mean precipitation increase (Fig. 2,
bottom). The data is the monthly Global Historical Climate Network data available at
http://www.ncdc.noaa.gov/ghcnm/v3.php
To date, these regional patterns are not well understood on physical grounds. This, and
other regional examples of climate trends, illustrate the need for a more comprehensive
assessment on the causes of regional and seasonal differences in climate trends that
considers multiple possible contributing factors, including atmospheric dynamics and
coupled ocean-atmosphere processes, land surface and biological processes, atmospheric
chemistry and aerosols, and human-induced climate change.
Summary
The certainty language expressed in the Hansen NYT piece about the coming dustbowl
fate for the Great Plains region and Midwest is contrary to the low confidence of regional
climate change projections for coming decades as documented in USGCRP and IPCC
reports. Not only are various regional patterns of trends that have been observed over the
last century poorly understood, but the projections of regional changes in coming decades
are highly uncertain.

4. Figure 2. The 1901-2010 trends in summertime (June-August) daily averaged surface temperature
(°C/110 yrs; top) and rainfall (% of change over 110 yrs, bottom). Trends are plotted at available
station sites, using the GHCNv3 data. Cooling (warming) trends shown in blue (red), and increased
(decreased) rainfall shown in blue (red).
The Hansen NYT piece asserts:
"The global warming signal is now louder than the noise of random weather..."
To which I replied:
“This is patently false. Take temperature over the U.S. as an example. The variability
of daily temperature over the U.S. is much larger than the anthropogenic warming

5. signal at the local, weather time scales. Depending on season and location, the
disparity is at least a factor of 5 to 10. I think that a more scientifically justifiable
statement, at least for the U.S. and extratropical land areas is that --- Daily weather
noise continues to drum out the siren call of climate change on local, weather
scales.”
Supporting Material for My Statement
Weather---as experienced on a daily basis and at any particular location--- is highly
variable, but the challenge offered in the Hansen NYT piece is that the noise of such
variations are now being drowned out by the global warming signal. Debating this
latter point should not, of course, be confused with opening a debate about the rise
in global mean temperatures. The IPCC (2007) has stated that warming of the
climate system is unequivocal. The unequivocal rise in global average temperature
and the attribution that most of this rise is due to the rise in anthropogenic
greenhouse gas concentrations, owes to the small natural variability of global mean
temperatures compared to the large magnitude of the human-induced warming
signal in global averaged temperatures. In other words, for global mean
conditions, the signal is much louder than natural variability of globally averaged
temperatures. The science is clear, we know that the planet has warming over the
past century, and we are very confident as to why such warming has (and
continues) to occur.
But that appears not to be the point of the NYT piece, as impliedby the subsequent
context of Hansen’s statement regarding extreme local weather events. The
“random weather” called out in the NYT piece is unlikely meant to refer to the
variability in global mean temperature, but rather to the local conditions we
regularly encounter in our own backyards and that swing back and forth across a
range of conditions. Thisrange (the random noise, as per Hansen) is in fact not
smaller than the global warming signal, as shown from several lines of evidence
below.
The recent published peer-reviewed study by Hawkins and Sutton (2012,
Geophysical Research Letters) diagnoses the so-called “time of emergence” of
climate signals from the noise of random variability at a local level. Their analysis
compares an estimate of a human-induced change in surface air temperature
against an estimate of its natural variability for seasonally averaged data. For a
signal-to-noise ratio of two (signal being double the magnitude of the noise), they
find that the time of emergence is after 2050 for most mid-latitude regions during
cold and warm seasons. The tropics, where noise of temperature variability is less
than in mid-latitudes, the time of emergence is appreciably earlier.
But even that analysis is not quite germane to the Hansen assertion regarding the
noise of random weather. Figure 3 presents an analysis of the intensity of the
variability of daily averaged surface temperature across the United States.

6. The data is daily temperature at Cooperative observing stations, which is available
at http://www.ncdc.noaa.gov/oa/climate/research/gdcn/gdcn.html. The top panel
is the intensity of daily temperature variability during 1901-2010, averaged across
the 12 calendar months January-December. On average, daily temperature
variability is about 5°C over North Dakota, to as small as 2°C over Florida. The
recent update, by NCDC, of the annual mean global mean warming signal is +0.51°C
(for 2011 relative to a 20th Century reference). It is thus evident that daily surface
temperature variability is on order 5 to 10 times greater than the global warming
signal (see Fig. 3, bottom). Consistent with the published work of Hawkins and
Sutton, it is obvious that the time of emergence of the global warming signal from
this weather noise is far in the future under the assumption of continuing global
warming.
Summary
The global warming signal is much smaller than the typical daily variability of surface air
temperature over the United States. Most of the magnitude of daily weather extremes
owes its causes to natural internal fluctuations and not to global warming. A possible
exception could be imagined if global warming were also to increase the variability of
daily temperatures (and not just increase the mean temperatures), but no compelling
evidence to such effects has been shown. While globally averaged temperatures have
risen during the past century, the cause for which is very likely human-induce climate
change, the signal of this change is still barely audible among the loud noise of daily,
backyard weather fluctuations.
Weather, of course, is more than temperature variability. While this discussion has
involved temperature, weather involves rain, storms, winds, severe convection, clouds
among others. In this regard, it is important to reiterate the statement in IPCC SREX
(2012) in their Executive Summary which states that “many weather and climate
extremes are the result of natural climate variability”, and that “even if there were no
anthropogenic changes in climate, a wide variety of natural and weather extremes would
still occur”.

7. Figure 3.The daily surface temperature variability during 1901-2010 averaged for all months during
January-December (°C, top), and the ratio of that daily variability to the magnitude of the observed
global warming signal (nondimensional). The variability is the standard deviation of daily
temperature fluctuations calculated for each calendar month, and averaged across all months. The
global mean warming signal of +0.51°C is derived from the NCDC analysis of the 2011 annual mean
global averaged surface temperature departure relative to a 20 th Century climatology (see
http://www.ncdc.noaa.gov/sotc/global/2011/13)
The Hansen NYT piece asserts:
" We can say with high confidence that the recent heat waves in Texas and Russia,
and the one in Europe in 2003, which killed tens of thousands, were not natural
events — they were caused by human-induced climate change."
To which I replied:
“Published scientific studies on the Russian heat wave indicate this claim to be
false. Our own study on the Texas heat wave and drought, submitted today to
Journal of Climate, likewise shows that that event was not caused by human-induced
climate change. These are not de novo events, but upon scientific scrutiny,one finds
both the Russian and Texas extreme events to be part of the physics of what has
driven variability in those regions over the past century. Not to say that climate
change didn’t contribute to the those cases, but their intensity owes to natural, not
human, causes.”

8. Supporting Material for My Statement
The principal conclusion by Dole et al. (2011, Geophysical Research Letters) is that
the extreme magnitude of the 2010 Russian heat wave was mainly due to internal
dynamical processes (associated with atmospheric blocking), and that it was very
unlikely that warming attributable to GHG forcing contributed substantially to the
heat wave's magnitude. Rahmstorf and Coumou (2011, PNAS) concluded that a
strong warming over western Russia (which they attribute primarily to GHG
forcing) multiplied the likelihood of a record heat wave. They estimated an 80%
probability that the 2010 July heat records in Moscow would not have occurred
without climate warming. Barriopedro et al. (2011, Science), on the other hand,
conclude that the magnitude of the 2010 event was so extreme that despite GHG
warming, the likelihood of an analog over the same region remains fairly low at this
time. This is consistent with estimates in Dole et al., which showed a very low
probability of an event of this magnitude in 2010, but a rapidly increasing likelihood
of crossing given thresholds in future climate, based on results from CMIP3 model
runs.
Toward attempting to reconcile these conclusions, Otto et al (2012, Geophysical
Research Letters) conclude that there is no substantial contradiction between
studies by Dole et al. and Rahmstorf and Coumou, in that the heat wave “can be both
mostly internally generated due to magnitude, and mostly externally-driven in
terms of occurrence-probability”. Further discussion on this matter including an
extensive list of recent published work on the Russian heat wave is available at
http://www.esrl.noaa.gov/psd/csi/events/2010/russianheatwave/index.html
Summary
Analysis of various forced model simulations indicates that human influences did
not contribute substantially to the magnitude of the Russian heat wave. Even
accounting for a possible stronger warming signal, as suggested by Rahmsdorf and
Coumou, these were still appreciably smaller than the peak magnitude of the event
(which reached 10°C over Moscow during July). Barriapedro et al. (2011) conclude
that the magnitude of the 2010 event was so extreme that despite an increase in
temperatures due to human climate change, the likelihood of an analog over the
same region remains fairly low until the second half of the 21st century. These
results are thus consistent also with the Hawkins and Sutton (2012) results
regarding the time of emergence of a climate change signal at local scales.