Climate change (4)

For completeness I will mention that a few more steps are required before adjusted station readings become the global average temperature. First, the surface of the Earth is divided into “grid cells” – geographical areas delineated by longitude and latitude co-ordinates. The average grid cell temperature is worked out from the (adjusted) readings from the weather stations located within the cell. The average temperature in cells with no weather stations in them is usually interpolated from the adjacent populated cells. The cell data is then processed to give the average global temperature.

So what is the global averaged temperature graph telling us?

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The graph indicates a general warming trend, but with a number of random-looking dips. The magnitude of the warming in the last 100 years is of the order of 0.8C. Many skeptics believe that the adjustments discussed in the previous post introduce some artificial warming (mainly through UHI) and the true rate of warming may be closer to 0.5-0.6C per century. Additionally, depending on the selected reference period, the rate of warming varies and can even be negative (when the global temperatures dipped in late 19th century and again in 1940-1970).

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Be that as it may, hardly anyone disputes that the global temperatures are, generally, on the way up. But how does the global temperature graph correlate with the supposed cause for the warming, atmospheric CO2?

co2_new

Not very well. The CO2 levels have been growing steadily through the 20th century, while the global temperatures first spiked between 1910-1940 and then dipped for three decades. The most dramatic discrepancy is after year 2000, when CO2 levels kept rising (not shown in the graph above) but the temperature plateaued. It is clear that factors other than CO2 affect the Earth climate but let us first assess the theoretical impact of the increased level of CO2, using the “greenhouse” physical model.

The theory of the greenhouse effect is well established and, in essence, describes how the heat radiated by the Sun gets trapped under the blanket of the Earth atmosphere. CO2 (and other greenhouse gasses) increase the effectiveness of the trapping, causing the rise in global temperatures. It is very easy to calculate the theoretical climb in global temperatures which should have resulted from the observed increase in the atmospheric concentration of the known greenhouse gasses in the last 100 years. It is actually somewhat higher than the rate of warming in the official global temperature record.

The scientific formulation of the above issue refers to the temperature increase from the doubling of CO2 in the atmosphere – also known as climate sensitivity. The Earth climate sensitivity as per the greenhouse theory is around 1.0C, the observed sensitivity (based on the global temperature record) is around 0.8C. So, the Earth’s response to the increase in the greenhouse gasses appears to be somewhat lower than predicted by theory. There is a number of theories where the missing heat has gone. Some possible explanations are that it is hiding in the oceans or that on the way to Earth surface it got stopped by clouds or aerosols. This, in climate-speak, is referred to as negative feedbacks.

Based on predictions we can expect a doubling of CO2-equivalent concentration of the greenhouse gasses by year 2100. Applying the above climate sensitivities we would project that the likely resulting increase in global temperatures is of the order of 0.8-1.0C. This should not cause too many problems – remember that we had a similar magnitude of warming in the last 100-150 years when the quality of life has improved considerably. So, one may ask, what is the problem? This brings me to the main (and possibly the ONLY) sticking point in the AGW debate.

Climate change (5)

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