Written by Hans Schreuder
Most climate models assume the greenhouse gas carbon dioxide CO2 to influence significantly upon global temperature. Thus, it is relevant to compare the different global temperature records with measurements of atmospheric CO2, as shown in the diagrams below. Any comparison, however, should not be made on a monthly or annual basis, but for a longer time period, as other effects (oceanographic, clouds, etc.) may well override the potential influence of CO2 on short time scales such as just a few years.
It is of course equally inappropriate to present new meteorological record values, whether daily, monthly or annual, as support for the hypothesis ascribing high importance of atmospheric CO2 for global temperatures. Any such short-period meteorological record value may well be the result of other phenomena than atmospheric CO2.
What exactly defines the critical length of a relevant time period to consider for evaluating the alleged high importance of CO2 remains elusive, and is still a topic for debate. The critical period length must, however, be inversely proportional to the importance of CO2 on the global temperature, including feedback effects, such as assumed by most climate models.
After about 10 years of global temperature increase, IPCC was established in 1988. Presumably, several scientists interested in climate then felt intuitively that their empirical and theoretical understanding of climate dynamics was sufficient to conclude about the importance of CO2 for global temperature. However, for obtaining public and political support for the CO2-hyphotesis the 10 year warming period leading up to 1988 in all likelihood was important.
Had the global temperature instead been decreasing, public support for the hypothesis would have been difficult to obtain. Adopting this approach as to critical time length, the varying relation (positive or negative) between global temperature and atmospheric CO2 has been indicated in the lower panels of the three diagrams below.
Diagrams showing HadCRUT3, GISS, and NCDC monthly global surface air temperature estimates (blue) and the monthly atmospheric CO2 content (red) according to the Mauna Loa Observatory, Hawaii. The Mauna Loa data series begins in March 1958, and 1958 has therefore been chosen as starting year for the diagrams.
Reconstructions of past atmospheric CO2 concentrations (before 1958) are not incorporated in this diagram, as such past CO2 values are derived by other means (ice cores, stomata, or older measurements using different methodology, and therefore are not directly comparable with modern atmospheric measurements.
The dotted grey line indicates the approximate linear temperature trend, and the boxes in the lower part of the diagram indicate the relation between atmospheric CO2 and global surface air temperature, negative or positive.
The above chart shows the Global Sea Surface Temperature. Noting that the beginning temperature and the final May 2009 entry both at the '0' base measuring anomaly, with the temperatures from early 2003 showing level and then beginning to decline (cool) 2006 through present.
Edited for I Love My Carbon Dioxide by Hans Schreuder, July 2009
Original Study Report by Ole Humlum
M.Sc., Ph.D., Professor of Physical Geography