In a recent letter challenging man-made climate change [“Climate change is natural,” Dec. 12, The Calvert Recorder], Mr. Phil Zalesak of Tall Timbers perpetuates some misunderstandings, which I trust the following will correct:

Mr. Zalesak seems to accept the evidence that climate is changing, but thinks it’s due to some natural cause. Unfortunately, he does not offer any such natural cause. The fact that climates have changed in the distant past is not by itself an explanation for the ongoing changes. Yes, ice ages (and also “hothouse climates”) have come and gone, and computer simulations can, to a great degree, explain why they did. These simulations are hardly just “somebody’s computer model.” There are specialized teams of scientists and computer programmers, in the U.S. and elsewhere, using powerful computers and, I imagine, many thousands of lines of code. The models compete with each other (as the hurricane modelers did with Hurricane Sandy) to see which best predicts what is happening and what happened in the past. If anything, observations of Greenland glacier ice loss (it’s even accelerating, as a 2012 research paper suggests, based on two different earth satellite-based methods) and Arctic sea ice loss have shown that actual changes are happening even faster than predicted. No climate scientist would claim computer simulations can predict future climates with “absolute certainty,” but the various competing models, including those predicting sea level rise, are in general agreement for the near future (e.g. 2050). For the more distant future — say the year 2150 — the models diverge, and part of that divergence is due to uncertainty on exactly how much fossil carbon-based fuel is burned every year, which is a factor mankind can influence or mitigate. All models predict that carbon dioxide (unlike methane) stays in the atmosphere for a very long time: centuries to millennia.

Mr. Zalesak’s comparison between forecasting weather and forecasting climate is misplaced, due to the chaotic nature of weather. Next week’s weather will almost certainly never be predictable with 100 percent accuracy, unless it’s a part of the world where the weather never changes from day to day. Climate is average weather and, thus, is easier for computer simulations to predict. Of course, it takes time — years to decades — before the shorter-term fluctuations we call “weather” are averaged out and the models tested.

Mr. Zalesak is correct about the fact that greenhouse gases comprise only a tiny fraction of our atmosphere. Even carbon dioxide, the most abundant, makes up only about 391 parts per million (up from 280 ppm in pre-industrial times, and already well in excess of the highest — 295 ppm — in the last 800,000 years), but is steadily rising (no one disputes that this rise is largely or entirely manmade). However, the main gases in our atmosphere (nitrogen and oxygen) are transparent, not only to visible light, but also to infrared radiation. The visible light we get from the sun, clouds and smoke or dust permitting, is absorbed by the earth (making your car and the pavement very hot on summer days). This heat is radiated back up as infrared, but some is absorbed by greenhouse gas molecules, which then share the extra energy with surrounding nitrogen and oxygen molecules. The point is that even a small amount of greenhouse gas has a powerful effect, trapping heat — a fact that has been known for more than 100 years. If mankind pushes the level over 1,000 ppm, most models predict climates as warm as anything the world has seen in tens of millions of year.

I can recommend an excellent, beautifully illustrated climate textbook (no math): “Earth’s Climate — Past and Future,” by Prof. William F. Ruddiman, W.H. Freeman and Co., New York. Bill is retired from the University of Virginia, but lived in downtown Prince Frederick in the 1970s, when he and I both worked for the U.S. Naval Oceanographic Office. No politics are in his book. I urge those who are interested in this important topic to read the relevant parts of this or another climate book before taking a position on the subject of climate change.

Peter Vogt, Port Republic