The Conversion of a Climate-Change Skeptic
New York Times
29 July 2012
By Richard A. Muller
CALL me a converted skeptic. Three years ago I identified problems
in previous climate studies that, in my mind, threw doubt on the
very existence of global warming. Last year, following an
intensive research effort involving a dozen scientists, I
concluded that global warming was real and that the prior
estimates of the rate of warming were correct. I'm now going a
step further: Humans are almost entirely the cause.
My total turnaround, in such a short time, is the result of
careful and objective analysis by the Berkeley Earth Surface
Temperature project, which I founded with my daughter Elizabeth.
Our results show that the average temperature of the earth's land
has risen by two and a half degrees Fahrenheit over the past 250
years, including an increase of one and a half degrees over the
most recent 50 years. Moreover, it appears likely that essentially
all of this increase results from the human emission of greenhouse
gases.
These findings are stronger than those of the Intergovernmental
Panel on Climate Change, the United Nations group that defines the
scientific and diplomatic consensus on global warming. In its 2007
report, the I.P.C.C. concluded only that most of the warming of
the prior 50 years could be attributed to humans. It was possible,
according to the I.P.C.C. consensus statement, that the warming
before 1956 could be because of changes in solar activity, and
that even a substantial part of the more recent warming could be
natural.
Our Berkeley Earth approach used sophisticated statistical methods
developed largely by our lead scientist, Robert Rohde, which
allowed us to determine earth land temperature much further back
in time. We carefully studied issues raised by skeptics: biases
from urban heating (we duplicated our results using rural data
alone), from data selection (prior groups selected fewer than 20
percent of the available temperature stations; we used virtually
100 percent), from poor station quality (we separately analyzed
good stations and poor ones) and from human intervention and data
adjustment (our work is completely automated and hands-off). In
our papers we demonstrate that none of these potentially
troublesome effects unduly biased our conclusions.
The historic temperature pattern we observed has abrupt dips that
match the emissions of known explosive volcanic eruptions; the
particulates from such events reflect sunlight, make for beautiful
sunsets and cool the earth's surface for a few years. There are
small, rapid variations attributable to El Niño and other
ocean currents such as the Gulf Stream; because of such
oscillations, the "flattening" of the recent temperature rise that
some people claim is not, in our view, statistically significant.
What has caused the gradual but systematic rise of two and a half
degrees? We tried fitting the shape to simple math functions
(exponentials, polynomials), to solar activity and even to rising
functions like world population. By far the best match was to the
record of atmospheric carbon dioxide, measured from atmospheric
samples and air trapped in polar ice.
Just as important, our record is long enough that we could search
for the fingerprint of solar variability, based on the historical
record of sunspots. That fingerprint is absent. Although the
I.P.C.C. allowed for the possibility that variations in sunlight
could have ended the "Little Ice Age," a period of cooling from
the 14th century to about 1850, our data argues strongly that the
temperature rise of the past 250 years cannot be attributed to
solar changes. This conclusion is, in retrospect, not too
surprising; we've learned from satellite measurements that solar
activity changes the brightness of the sun very little.
How definite is the attribution to humans? The carbon dioxide
curve gives a better match than anything else we've tried. Its
magnitude is consistent with the calculated greenhouse effect -
extra warming from trapped heat radiation. These facts don't prove
causality and they shouldn't end skepticism, but they raise the
bar: to be considered seriously, an alternative explanation must
match the data at least as well as carbon dioxide does. Adding
methane, a second greenhouse gas, to our analysis doesn't change
the results. Moreover, our analysis does not depend on large,
complex global climate models, the huge computer programs that are
notorious for their hidden assumptions and adjustable parameters.
Our result is based simply on the close agreement between the
shape of the observed temperature rise and the known greenhouse
gas increase.
It's a scientist's duty to be properly skeptical. I still find
that much, if not most, of what is attributed to climate change is
speculative, exaggerated or just plain wrong. I've analyzed some
of the most alarmist claims, and my skepticism about them hasn't
changed.
Hurricane Katrina cannot be attributed to global warming. The
number of hurricanes hitting the United States has been going
down, not up; likewise for intense tornadoes. Polar bears aren't
dying from receding ice, and the Himalayan glaciers aren't going
to melt by 2035. And it's possible that we are currently no warmer
than we were a thousand years ago, during the "Medieval Warm
Period" or "Medieval Optimum," an interval of warm conditions
known from historical records and indirect evidence like tree
rings. And the recent warm spell in the United States happens to
be more than offset by cooling elsewhere in the world, so its link
to "global" warming is weaker than tenuous.
The careful analysis by our team is laid out in five scientific
papers now online at BerkeleyEarth.org. That site also shows our
chart of temperature from 1753 to the present, with its clear
fingerprint of volcanoes and carbon dioxide, but containing no
component that matches solar activity. Four of our papers have
undergone extensive scrutiny by the scientific community, and the
newest, a paper with the analysis of the human component, is now
posted, along with the data and computer programs used. Such
transparency is the heart of the scientific method; if you find
our conclusions implausible, tell us of any errors of data or
analysis.
What about the future? As carbon dioxide emissions increase, the
temperature should continue to rise. I expect the rate of warming
to proceed at a steady pace, about one and a half degrees over
land in the next 50 years, less if the oceans are included. But if
China continues its rapid economic growth (it has averaged 10
percent per year over the last 20 years) and its vast use of coal
(it typically adds one new gigawatt per month), then that same
warming could take place in less than 20 years.
Science is that narrow realm of knowledge that, in principle, is
universally accepted. I embarked on this analysis to answer
questions that, to my mind, had not been answered. I hope that the
Berkeley Earth analysis will help settle the scientific debate
regarding global warming and its human causes. Then comes the
difficult part: agreeing across the political and diplomatic
spectrum about what can and should be done.