After following the climate discussions for several years, I find that the term CAGW (Catastrophic Anthropogenic Global Warming) brings together the four issues that keep coming up.

1. Is there warming around us?
2. Is the warming global (everywhere)?
3. Is warming caused by human activity?
4. Is warming catastrophic now or likely to become so?

Here’s my take of the state of the discussion:

1. The instrumental records show a pattern of warming, with accelerating and decelerating phases. The amount of warming depends upon the time periods selected for comparisons. The measurements of warmings are in decimals of degrees, with error ranges of a size to significantly impact on the results. The records have been subjected to various adjustments for various reasons, and not all of these have been verified to prove the adjusted numbers are more reliable. Many people accept that we are in a modern warming period, though there are uncertainties with the measurements. No additional warming has occurred this century, and this lack of warming may signify future cooling.

2. Global averages have been produced and they show a warming trend. These are averages of anomalies, since the actual temperatures vary greatly according to both place and time. Again the selection of the baseline (normal) for comparison affects the results. There is great diversity of warming and cooling patterns around the average; for example, at least 1/3 of US land surface stations showed cooling trends over the same period that the average was rising. Also, patterns in the mostly oceanic Southern Hemisphere (SH) are quite different and the average lower than the NH, where most of the land is. Often, two microclimates differ significantly even when a few KM apart, so that the selection of stations affects greatly the results. This issue is open to debate and is currently subject to extensive investigation.

3. Human activity directly impacts the environment and climate through land and water use: the effects of urban settlements, forest clearing, water extraction, damming of rivers, etc. appear to cause changes and often warming the places where they occur. The major debate is over the claim that burning of fossil fuels causes global warming by the increasing presence of CO2 in the atmosphere. Many issues are involved: Does rising CO2 cause rising temperatures, or the other way around? How much do emissions from humans affect the atmosphere considering the much larger exchanges between natural CO2 sources and sinks? How much does the radiative effect of 400ppm CO2 affect the climate, considering the effects of convection, evapotranspiration, multi-decadal ocean heat oscillations, cloud patterns, among other factors?

4. How dangerous is the present pattern of climate change, defined by IPCC as manmade global warming? The extent of human contribution to observed warming is uncertain. Even so, the modern warming period was preceded by the Medieval, the Roman, and the Minoan warming periods–each was cooler than the previous, and all of them warmer than the present. The last 1.5C of warming has been a boon to human agriculture and civilization, and the next 1.5C is likely to also be beneficial. Yet numerous studies are funded to examine any and all negative consequences that could result from increases in temperature. The funding monopoly dedicated to climate change ensures a steady drumbeat of warnings. The public’s concern is required for governments to impose carbon-pricing regimes as the proposed means of reducing CO2 emissions. Many doubt whether these regimes will reduce either CO2 or warming. Some believe that natural forces are already beginning to cool the climate, in spite of emissions. This is a battle for public opinion waged daily in the media and the blogger sphere.