This is the fourth in a series on the attribution of climate change; that is, how do we determine to what extent the observed warming is caused by humans? The earlier entries are cataloged at the end.
In the previous two entries temperature, the obvious measure of global warming, was discussed. Looking just at temperature one is faced, head on, with all forms of variability. I am in Boulder, Colorado for the summer, and here, there are huge changes in temperature from day-to-day for much of the year. One day there is air from the Gulf of Mexico, and the next day there is air from the high mountains to the north and west. More generally, there is variability due to natural oscillations like El Nino – La Nina
and the North Atlantic Oscillation
; there is variability due to changes in how land is used. Temperature variability is also tied up with water, especially for some of the proxy data that represent temperature from before the time of thermometers.
From the observations, we are, in the late 20th and early 21st century, living in a time of high, sustained global temperatures. There have been other periods of sustained warm temperatures in recorded history, notably the Medieval Warm Period. (Here is an interesting collection of papers from 1994 on the Medieval Warm Period
, and a paper
from my class web site.)
There is evidence, however, that the warming in the current period is the highest above the average than at any time in the past 1000 years. ( Attribution (3)
) Plus, the warming is evident in more and more data sets. This does not mean that all observations, at all times, at all places will be warmer than long-term averages. To have that expectation does not recognize the natural variability associated with weather. Citing single examples of cooling does not mean that global warming is not occurring. Arguments that rely on citing examples of below normal temperatures require the establishment that the scope and magnitude of that signal is relevant to the globe as a whole. In fact, on a planet that is on average becoming warmer at the surface, some parts of the planet will, in fact, be colder; it is conservation of energy. Still, the reliance on surface temperature records, alone, is not adequate to establish that the Earth is warming and why it is warming.
There are many ways to investigate the causes of warming. In the absence of the “smoking gun,” it is the pursuit of multiple paths of investigation that provide confidence in attribution. One of the most compelling paths to investigate attribution is the idea of “fingerprinting.” The basic idea of fingerprinting is exemplified by the following figure. (I want to acknowledge and thank Ben Santer
at Lawrence Livermore National Laboratory for teaching me all I know about “fingerprinting” and determining attribution.)
Figure 1. Comparison of the altitude and latitude changes in thermal structure of the atmosphere related to increases in solar forcing (left) and increases in CO2 (right). W is warming, C is cooling. The relative size of warming is indicated by the size of the letter. The altitude extends from the surface to the middle of the stratosphere. The tropopause, which is the top of the troposphere and the “boundary” between the troposphere and the stratosphere, is shown as a dashed line. (This is adapted from Temperature Trends in the Lower Stratosphere
, one of the Synthesis Report’s from the U.S. Climate Change Science Program
In this figure on the left is a qualitative diagram of the signal calculated from theory and models associated with an increase in solar heating. There is warming at the surface, more warming at the North Pole that at the South Pole. At the poles, in the stratosphere is cooling. In the stratosphere in the middle latitudes and the tropics there is warming. There is warming in the tropical upper troposphere, just below the tropopause. On the right of the figure is the heating that would occur if the solar heating was constant and there was a carbon dioxide increase. Of greatest importance for attribution is the strong cooling that occurs in the stratosphere for a carbon dioxide increase. Hence, there is a distinct difference between the spatial distributions of warming and cooling that come from a change in solar heating compared with a change in greenhouse gases. This introduces the basic concept of fingerprinting; when we look at the expected spatial and temporal variability of change, warming or cooling, that comes from a possible change in the environment are there characteristics that allow the attribution of the source of the change?
There are other sources of change in the heat balance of the planet. Changes in ozone, changes in aerosols due to volcanoes, changes in land use (albedo), etc. Each of these changes has a distinct signal. Of course, in the real atmosphere, all of these changes are present. Therefore, “fingerprints” that are more complete than the example presented here need to be developed.
WU blogs on Attribution of Climate Change to Human Activities: WU Blog on Models and Attribution Attribution (1) Attribution (2) Attribution (3)