"My overall view of the influence of humans on climate is
that we probably are having some influence, but it is impossible to know with
any level of certainty how much influence. The difficulty in determining the
human influence on climate arises from several sources:
(1) weather and climate vary naturally, and
by amounts that are not currently being exceeded;
(2) global warming theory is
just that – based upon theory; and
(3) there is no unique fingerprint of human caused global
warming.
My belief that some
portion of recent warming is due to humans is based upon my faith in at least some portion of the theory: that the human
contribution to atmospheric greenhouse gas concentrations has resulted in an estimated
1% reduction in the Earth’s ability to cool to outer space, and so some level of warming
can be expected to occur from that change
Exactly how much warming will occur, however, depends upon
something we call “climate sensitivity” (Spencer & Braswell, 2010; 2011),
and relatively few researchers in the world – probably not much more than a dozen – have
researched how sensitive today’s climate system is based upon actual
measurements. This is why popular surveys of climate scientists and their
beliefs regarding global warming have little meaning: very few of them have
actually worked on the details involved in determining exactly how much warming
might result from anthropogenic greenhouse gas emissions.
Our most recent peer-reviewed paper on this subject, Spencer
& Braswell (2013), has arrived at a climate sensitivity of only 1.3 deg. C for
a doubling of atmospheric carbon dioxide, based upon a variety of global
measurements, including warming of the global oceans since the 1950s. This
level of warming is below the lower limit of 1.5 deg. C minimum predicted in
the last (AR4) IPCC report. It is also in line with (an admitted minority of) other estimates of low climate sensitivity
published in the peer review literature.
It should also be noted that the fact that I believe at
least some of recent warming is human-caused places me in the 97% of researchers recently
claimed to support the global warming consensus (actually, it’s 97% of the published
papers, Cook et al., 2013). The 97% statement is therefore rather innocuous,
since it probably includes all of the global warming “skeptics” I know of who are actively working in the
field. Skeptics generally are skeptical of the view that recent warming is all
human-caused, and/or that it is of a sufficient magnitude to warrant immediate action given the
cost of energy policies to the poor. They do not claim humans have no impact on climate
whatsoever."
Extract from the testimony made by Roy W. Spencer on 18th July 2013 to the Environment and Public Works Committee of the US Senate.
His statement includes this description of his relevant background: "I have been performing U.S. government-sponsored research for the last twentyeight years, publishing peer reviewed papers on global temperature monitoring with satellites, on the amount of warming we might expect from greenhouse gas emissions, how to monitor hurricane strength from satellites, and quantitatively explaining ocean heat content changes.
Prior to my current position as a principal research scientist at the University of Alabama in Huntsville, I was Senior Scientist for Climate Studies at NASA’s Marshall Space Flight Center. I am also the U.S. Science Team Leader for the Advanced Microwave Scanning Radiometer-E flying on NASA’s Earth-observation satellite Aqua. I am a recipient of NASA’s Medal for Exceptional Scientific Achievement."