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Letter to the Editor: Brief Response to 'Weak Correlation Between CO2 and Temperature ...'

on August 3, 2013 - 8:28am
By Stephen Price and Charles Keller
Los Alamos
We have responded briefly to the writer’s comments, which are in quotes, and written additional comments of our own. We are happy to provide scientific journal citations to support any of our statements made below.

“A weak short-term correlation between CO2 and temperature proves nothing about causation. Proponents of the notion that increases in the air's CO2 content lead to global warming point to the past century's weak correlation between atmospheric CO2 concentration and global air temperature as proof of their contention.”

Actually they don’t. But they do show that physically based computer climate simulations, forced in part by observed changes in greenhouse gases (like CO2), agree well with observations of temperature changes in the past.

“... [proponents of global warming] typically gloss over the fact that correlation does not imply causation, and that a hundred years is not enough time to establish the validity of such a relationship when it comes to earth's temperature history.”

We agree that correlation does not prove causation, but it does lead to a search for such causality and there has been an overwhelming finding of that. In addition, there is good reason to search for causality in this case. It has been know since the mid-to-late 1800’s, from very basic theories of physics and chemistry and through the work of scientific greats like Fourier, Tyndall, and Arrhenius, that greenhouse gases like CO2 increase the temperature of earth’s atmosphere (earth would be approximately 30 deg C [~90 deg F] colder without them). Laboratory experiments have confirmed these theories and it stands to reason that increasing the concentration of these gases in the atmosphere will increase the atmospheric temperature further. Correlation does not imply causation, but in this case very basic, long-standing science does argue for causation.

In addition, scientists don’t rely solely on the temperature record of the past hundred years. There has been considerable study and simulation of proxy records of global temperature and greenhouse gas changes going much farther back in time. These studies also support the notion of causation.

“The observation that two things have risen together for a period of time says nothing about one trend being the cause of the other. To establish a causal relationship it must be demonstrated that the presumed cause precedes the presumed effect. Furthermore, this relationship should be demonstrable over several cycles of increases and decreases in both parameters. And even when these criteria are met, as in the case of solar/climate relationships,”

Correlations between solar variability and climate are not all that good, but physical studies do show a weak influence of the small, periodic brightening and dimming of the sun on global temperatures. Scientists take this into account when determining the climate system response to increasing CO2.

“In thus considering the seven greatest temperature transitions of the past half-million years - three glacial terminations and four glacial inceptions - we note that increases and decreases in atmospheric CO2 concentration not only did not precede the changes in air temperature, they followed them, and by hundreds to thousands of years!”

This statement is simply not true. Consider the rise in temperature after the last Glacial Maximum. CO2 rose nearly at the same time. Until recently, dating was not precise enough to say which was first. A recent study, however, shows that CO2 rose nearly simultaneously, or perhaps a little before temperature. When CO2 is observed to increase after the initial rise in temperature, this still agrees with scientists understanding of the climate system; orbital rotational cycles of the Earth would start the warming (through small changes in earths radiation budget), which causes CO2 to come out of solution from warming oceans and into the atmosphere. This creates a positive feedback that magnifies the small initial warming. In fact, the CO2 feedback is responsible for most of the subsequent warming (the influence of orbital changes alone on earth’s radiation budget are generally too small to account for the magnitude and rate of the observed warming coming out of an ice age).

“There were also long periods of time when atmospheric CO2 remained unchanged, while air temperature dropped, as well as times when the air's CO2 content dropped, while air temperature remained unchanged or actually rose. Hence, the climate history of the past half-million years provides absolutely no evidence to suggest that the ongoing rise in the air's CO2 concentration will lead to significant global warming.”

This statement is also untrue. We ask that the author provide citations to articles from refereed science journals to support this claim.

In general, there is a fallacy in the writer’s reasoning. He says (correctly) that correlation doesn’t prove causation until we have physical evidence. And so he should also hold to that principle in the reverse case and admit that neither does anti-correlation disprove causation, at least until we more fully understand the dynamics of the climate system.

When we look for physical evidence for CO2-caused warming, we find it in so many ways that there is little room for taking the contrary position. In fact, even most professional critics do not disagree that CO2 has caused much of the observed warming. Instead, they argue that the sensitivity of climate to net CO2 forcing (causing warming) is less than is generally accepted. That argument proceeds as follows. A doubling of CO2 (relative to preindustrial times) would cause about 1.3 to 1.5 degrees Celsius warming, which leads to warmer oceans and more evaporation of water vapor, which itself causes more warming (because water vapor is also a potent greenhouse gas). This and other positive feedbacks would cause a further ~1.5°C warming. Critics point to evidence that CO2 warming also causes some negative feedbacks, such as increased cloudiness, which, for example, would cause cooling by reflecting incoming sunlight. Thus, rather than a climate sensitivity to CO2 doubling of ~3°C it may “only” be ~2°C.

Scientists may still be quibbling about some of the finer details, but the fact that the climate will continue to warm as CO2 in the atmosphere continues to increase is beyond a doubt.