(The following is an excerpt of the overview section of a paper by James Hansen, Reto Ruedy, Makiko Sato, and Ken Lo. The full paper can be read here.)
Public skepticism about global warming was reinforced by the extreme cold of December 2009 in the contiguous 48 United States and in much of Eurasia. The summer of 2009 was also unusually cool in the United States. But when a cold spell hits, we need to ask:
- Cold compared to what. Our memory of the past few winters? Winters of our childhood? Winters earlier in the 20th century?
- Cold where and for how long? Regional cold snaps are expected even with large global warming. Weather fluctuations can be 10, 20 or 30 degrees, much larger than average global warming.
- The reality of seasons. As the plot of Earth we live on turns away from the sun, in
winter or at night, it cools off. That's true even with global warming, albeit not quite so much.
Before addressing these matters, we note that scientists reporting global warming have come under attack for a supposed conspiracy to manufacture evidence of global warming. Perhaps because some members of the public accept these charges as reality, vicious personal messages are sent to the principal scientists almost daily.
The spiral into an almost surrealistic situation with ad hominem attacks on scientists may have originated in part with vested interests who do not want society to address climate change. But there is more than that – including honest, wishful thinking that climate change is not really happening. But wishing does not alter facts.
The scientific method practically defines integrity.
[Albert Einstein: "The right to search for truth implies also a duty; one must not conceal any part of what one has recognized to be true."
Richard Feynman: "The first principle is that you must not fool yourself – and you are the easiest person to fool."
All scientists make honest mistakes, but the scientific method is designed to correct them. The skeptical nature of the scientific method causes conclusions to be reexamined as new data appears. Cases of deliberate fudging of data, of scientific fraud, are so rare that these infrequent episodes live in infamy for decades and even centuries.
We know of no cases of fraud in analyses of global temperature measurements. Despite unfounded accusations, we believe that our best approach is simply to continue to report our scientific results as clearly as possible. Most of the public continue to respect scientists for what they do and how they do it. We presume that most of the public can separate science from political commentary.
Our data show that 2009 was tied for the second warmest year in the 130 years of near‐global instrumental measurements – and the Southern Hemisphere had its warmest year in that entire period. Before discussing these data, and their reconciliation with regional cold anomalies, we must consider the time frame of comparison.
If we look back a century, we find cold anomalies that dwarf current ones. The photo reproduced above shows people walking on Niagara Falls in 1911. Such an extreme cold snap is unimaginable today. About a decade earlier, in February 1899, temperature fell to ‐2°F in Tallahassee, Florida, ‐9°F in Atlanta, Georgia ‐30°F in Erasmus, Tennessee, ‐47°F in Camp Clark, Nebraska, and ‐61°F in Fort Logan, Montana.The Mississippi River froze all the way to New Orleans, discharging ice into the Gulf of Mexico.
As we will show, climate is changing, especially during the past 30 years. The changes are perceptible, even though average temperature change is smaller than weather fluctuations. The answer to the simple question: "How come it's so damned cold" turns out to be simple: "Because it's winter."