Ed Caryl has submitted another essay, which indicates that CO2 is not as strong a driver as many would like to have us believe.
CO2 is Cool!
By Ed Caryl
The global warming amount if CO2 doubles has been a bone of contention for the last 30 years. The IPCC has settled on figures in the range from 1.5 to 4.5 degrees, with a most probable rise of 3°C. One of the figures widely quoted is based on CO2 rising from 295 ppm in 1900, to 365 ppm in 2000, while temperature rose 0.57 degrees. (For those readers allergic to math formulas, apologies are offered, but keep reading, the result is what is important, and the math is done for you.) Because the CO2 affect is agreed by most to be logarithmic, the formula for the temperature rise if CO2 doubles, when based on CO2 concentration and temperature rise over time, is:
ln2/(ln(CO2 at end of period/CO2 at beginning of period))/(the change in temperature). (ln is the
natural logarithm). Substituting the numbers from above, we get:
ln2/(ln(365/295)/0.57) = 1.85°C
This formula was used to calculate the CO2 doubling-temperature (the temperature rise if CO2 in the atmosphere doubles) over a slightly longer period, from 1880 to 2010, using the Law Dome (Antarctic ice samples) and Mauna Loa Hawaii CO2 levels spliced together, and the GISS (Goddard Institute for Space Studies) global surface temperature figures.
The Law Dome and Mauna Loa CO2 data overlap from 1960 to 1978, and agree quite closely over those years, so splicing them seems quite valid. First, here (from the sources above) is the temperature and the CO2 plotted together:
Figure 1. Plot of GISS global temperature anomaly and atmospheric CO2.
The result of CO2 doubling using the above formula with the beginning and end numbers in Figure 1 is:
ln2/(ln(389.78/290.7)/0.91 = 2.15°C. This is higher, but still less than the IPCC estimate of 3°C.
But what if we take different time periods for the calculation? A shorter period 50-year calculation results in a noisy chart because there is great variation in temperature from year to year, with negative as well as positive temperature changes, and in the years before 1970, the CO2 rise was very slow so the ratios are small numbers. Here is the plot of the CO2-doubling temperature rise using the above formula with a sliding 50-year window beginning with the period from 1880 to 1930 and ending with 1960 to 2010:
Figure 2. CO2 sensitivity over time using a 50-year window applied to the data in Figure 1. The red trace is a 10-year moving average on the calculated sensitivity. The black line is the linear trend.
The calculation results in large positive and occasionally negative numbers when a shorter period is used. Non-CO2 influences are visible, the warming in the late 30s and 40s, and the later cooling and warming again in the 50s 60s and 70s. These are visible in Figure 1, above. The Atlantic Multi-decadal Oscillation (AMO), the Pacific Decadal Oscillation (PDO), and others, cause these temperature cycles. These 60 to 70-year ocean temperature cycles will increase a sensitivity figure based on a 100-year window.
Volcanoes, La Niña, and El Niño events and above described ocean cycles, as well as the solar cycle, affect the plot because they perturb the system away from equilibrium. Anything that adds or subtracts from the temperature is indistinguishable from the affects of CO2. CO2 is not the only thing affecting temperature. Many people have forgotten this simple fact. The plot in Figure 2 is really the total climate sensitivity, not just from CO2.
The figure widely used for CO2 doubling is only stable if the window used is long. It is only accurate if other factors are not pushing the numbers up. If shorter periods are taken, wildly different numbers result, both positive and negative. In recent years, the numbers settle to just over 1.0°C because the CO2 differences over the 50-year windows are getting larger. What is the real Climate Sensitivity? It appears to be less than 1.0°C for CO2 doubling.
Keep in mind that these plots were generated using GISS global surface temperature data. This data has been criticized for including too many sites influenced by urban warming and for being adjusted upward in recent years, and downward for earlier years. This seems to be the case, as any upward bias would tilt the above plot in the upward direction, as is seen at the end of the plot.
In an effort to check for this, the same formula was used on UHA satellite temperature data for the last 31 years. If the window is the 30 years from 1979 to 2009, the result is 1.75°C, if from 1980 to 2010, the answer is 2.05°C. Again, a short window gives a noisy answer. If 2011 is cooler than 2009, the calculation will be less than 1.75°C. Caution must be used when selecting data for these calculations. As an example, a calculation using the window from 1980 to 2008 results in a negative –0.2°C sensitivity.
Here are the temperature and CO2 plots:
Figure 3. Satellite temperature and Mauna Loa CO2 plots.
The sensitivity was plotted using a sliding 20-year window on the 31 years of satellite data available. Here is that result:
Figure 4. CO2 sensitivity using satellite temperature data and a 20-year sliding window.
The plot is very noisy with two points going negative. We will really need 20 or 30 more years of satellite temperature data to see a valid result, but the 30-year window suggests that the sensitivity as measured by the satellite data will still be less than 2°. Keep in mind that many of the ocean cycles, such as the AMO and PDO, are on the order of 60 years in length, and recently have been in their positive phases. The satellite data is only half this long.
In summary, all these plots show that CO2 sensitivity is probably 1°C or less for a doubling of CO2. We will need a few more years of good temperature data to pin that down.
Meanwhile, all the people claiming sensitivities of 3° or more need to calm down. The above plots and calculations rule that out.
A CO2 climate sensitivity of 1°C for CO2 doubling is not very important. Also remember that this figure includes all the supposed positive feedbacks, because if they exist, they have had an influence on the temperature already. Keep in mind that CO2 will probably never double in the atmosphere for the simple reason that we will run out of easily available fossil carbon long before then. But this is fodder for the next article.