Tuesday, February 19, 2013

Global Warming Update: What to Make of the Data

First, it is undeniably true that humans are injecting carbon dioxide and soot into the atmosphere at a record pace.  The data are uncontested.  Second, it is undeniably true that CO2 is a “greenhouse gas” which inhibits radiation of Earth’s surface heat into space and thus has a global warming effect.  Third, it is well established that water vapor has a far stronger warming effect than CO2.  As a further complication, clouds made by the condensation of that water vapor can lead to either cooling or heating, depending on the density, altitude, and particle sizes in the clouds.  Thus we are forced to estimate what effect warming by CO2 will have on the water vapor (and cloud) content of the atmosphere, a very difficult task.  Fourth, we must also come to grips with the warming effects of soot and carbon black, which also are products of human origin via everything from biomass cooking fires to coal-fired power plants to diesel engines.  Fifth, we need to understand all the correlates of natural processes such as solar variability and volcanic dust emission.  We can see clearly from the data in the HADCRUT4 graph in the previous post (Global Warming Update) that warming (and cooling) of Earth is far more complex than any one factor can explain: attributing all the warming in any time interval to CO2 makes CO2 appear more important than it really is, and biases all predictions in the direction of exaggerated warming.

Temperatures are influenced by the amount of radiation absorbed by gases—but not in a linear fashion.  The temperature increase caused by multiplying the abundance of a gas such as CO2 by, say, a factor of two is called the “climate sensitivity”: temperature is related to the logarithm of the absorbing gas abundance.  Doubling the CO2 abundance from the 19th-century level of less than 300 parts per million (ppm) to about 600 ppm would have the same warming effect as doubling the CO2 pressure again, to 1200 ppm.  So what is this “climate sensitivity”?  Climate modelers have used numbers ranging from about 1.5 to 6.5 oC per doubling of CO2.  Current wisdom favors a number near 1.5 or 1.6, right at the very bottom of the range used for generating dire climate predictions, for the short-term effects of solar heating.

Prof. Berntsen in a previous post suggested that the rapid warming of the 1978-1998 time period was due to a random combination of natural factors, carbon dioxide warming, and soot warming.  If we wrongly attribute all the observed warming to CO2, we are led inevitably to a gross overestimate of its warming power, predicting unreasonably high “climate sensitivity” and leading computer models to exaggerate the future warming trend.  If Prof. Berntsen’s estimate holds up, the climate sensitivity of CO2 after taking out the effect of soot is only about 60% of 1.5-1.6 degrees: call it 1 ºC.

How would we describe the temperature graph in the previous post without making reference to theories and explanations?  We could break the graph up into five “eras”: 1850 to 1927, with no significant net temperature change and a temperature anomaly of -0.3 oC; 1927 to 1940, with a warming of about 0.3 oC; 1940 to 1978 with gentle cooling of 0.15 oC; 1978 to 1998 with a strong warming of 0.65 oC, and 1998 to the present, with no significant change.  The “noise” in the data is striking: there are many independent effects of similar size at work, which sometimes work in synchrony.  Of course, if we included data extending back to the “Little Ice Age” of the 1600s, all of the data on this graph would be termed “very warm”.  And if we were to reach back to the “Roman Warm Period” 2000 years ago we would find temperatures closely similar to those of today.  Going back 9000 years to the early Holocene (the present interglacial period) we would have found an Earth that was warmer than today without any human influence or record-high CO2 content, powered solely by natural variability.

It is sobering to realize that most of the “noise” in the temperature graph is not random measurement errors, but real climate effects that are not adequately treated in (and were not predicted by) present models.  But remember that, no matter how complex our modeling of the atmosphere, some important factors such as volcanic emission of dust and sulfur gases and the effects of the variability of solar activity and solar wind strength will still defy prediction. 

Climate modeling is one of the most difficult computational problems known.  Like any science, the body of available data expands rapidly, and computer models must constantly be updated to include those data.   Many effects, such as the role of clouds or of soot, or the variability of the Sun, are recognized as important factors even while we still lack the detailed quantitative understanding of them that we would need to incorporate them into computer models.  Critical thought is the essence of science: we learn from experience and constantly improve our theories in the light of new data.  Similarly, theory points out what data we need to improve our understanding, and may even suggest how to go about acquiring them.  Skepticism is not the enemy of science; it is the very heart of the scientific endeavor.

We need an end to name-calling and personal attacks and threats.  We need to remove the discussion of global warming from the realm of politics and economically involved interest groups of both extremes.  We need to accept that anthropogenic global warming is not a “settled science”, but a vigorous and ambitious area of research in which new knowledge is of critical importance.  Remember that Newtonian physics was once a “settled science”: and then along came Einstein.  For about a century, celestial mechanics was also viewed as “settled science”: then along came spaceflight and modern mathematics.  Climate science is neither “settled” nor “fraudulent”: we must stop repeating and amplifying the most strident rhetoric, very little of which emanates either from scientists or from those in the media with real understanding of the issues.  We need less activism and more understanding.

There is one final very simple point to make: the phenomena of nature are incredibly complex.  Simplistic slogans such as “big industry is destroying our planet” and “climate science is a left-wing plot” are not only ignorant; they endanger our future.  Let’s bury that simplistic rhetoric and strengthen the science of complexity.

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