Introduction to the Observations of Anthropogenic Global Warming
Posted by Ari Jokimäki on July 28, 2009
The theories on AGW (and the details of climate models) are based on huge amount of observations made in laboratories and in field (in atmosphere, oceans, land, space). The history of the subject is given in “The Discovery of Global Warming” by Spencer Weart. Links to the basics of the subject are given in this RealClimate article.
Observational side of the subject starts with laboratory measurements of the basic properties of so called greenhouse gases (GHG). Especially relevant are the infrared absorption properties of the molecules of GHG’s because it is the infrared radiation that GHG’s absorb in the atmosphere to create the greenhouse effect. There are hundreds, or more likely thousands, of studies made starting from 1850’s studying the infrared properties of the GHG’s. In a very basic level, the laboratory studies are done so that infrared radiation is directed through a concentration of certain gas, and then it is measured how much infrared radiation comes out to see if the gas is absorbing some of the radiation. There are lot of variations of this basic setup in different studies; some use different temperature, pressure, gas concentration, spectral range, and so on.
In addition to measuring the basic properties of the GHG molecules in laboratories, we have made lot of observational work of their properties in actual atmosphere. Our basic site for measurements is of course Earth’s atmosphere, but we have done lot of measurements of the atmospheres of other planets also (most relevant to the subject here are probably Venus’ and Mars’ atmospheres). Earth’s atmospheric measurements have been done by using airplanes, balloons, and satellites to take necessary instruments high in the atmosphere (or outside of it), and some measurements can be done on ground. These measurements have shown that greenhouse gases work in the atmosphere pretty much like we expect based on the laboratory measurements. Currently, we already have almost direct measurements of the warming caused by greenhouse gases. We also know from the observations that CO2 concentrations are rising in the atmosphere, and that the reason for the rising concentration is emissions from human activities (this can be deduced by measuring the fractions of certain carbon isotopes in atmosphere).
Further support for direct laboratory and atmospheric measurements of greenhouse gas effects comes from paleoclimatic records. There are some places where information of past climatic conditions can be found. Ice cores for example are one such place (there are many others as well). When more ice is created, some air bubbles are trapped within the ice. Those air bubbles therefore contain the air from the atmosphere as it was when the ice was created, and we can measure for example CO2 concentration from them. The ice cores drilled from the Antarctic for example contain air bubbles with atmospheric information reaching back hundreds of thousands of years. These records of past climate have teached us that greenhouse gases have had a strong effect to past climate.
From other perspective, for a long time we have kept temperature measurement records. Those records show that most of the 20th century Earth’s surface was getting warmer, just as would be expected if greenhouse gas concentrations would rise. Supporting that, we have lot of indirect methods to determine temperature. These have caused lot of debate, especially tree rings. There are also signals of warming, such as glacier melting and different effects in ecosystem (spring activity occurring earlier for example). Ocean temperatures have also been measured for a long time, and anthropogenic global warming signal has been found from the global ocean too.
We also do lot of measurements of the Sun, clouds, rain, volcanoes, ocean currents, wind, snow cover, and all such things that can have an effect to the weather and climate. There are also some special observations. Fossilized plants exposed by melting glaciers for example tell us that the current temperature is higher than it has been for thousands of years. All in all, the whole issue is very well covered by observations alone.
Later, I will be making more detailed entries on the issues discussed above (with decent references too), but for now I just leave you with this introduction to the subject.