We all use graphic images – charts, tables, cartoons, pictures – to illustrate and, hopefully, clarify the information we are trying to convey and the points we are trying to make. We also use them to help convince you of our point of view. As a reader or listener, you have to be careful not only to evaluate the written or spoken words, but to make sure the images used do not mislead you.
You have probably heard or read that climatologists have concluded the CO2 is responsible for most or all of the increase in average temperatures from 1980 to the present. So, let me start with a recent presentation I attended where a graph similar to the one at left was shown. This graph shows the difference in temperatures for 1998-2011 (in Celsius) compared with average temperature for the period 1950-1980 (click on image for a larger version). You can see that temperatures on this graph range from slightly above 3 to slightly more that 6 degrees Celsius above the 1950-1980 average temperature.
The presenter claimed that temperature and CO2 had become “uncoupled,” that is, that the kind of rising temperatures that we had experienced in the 1980’s and 1990’s have leveled off and that CO2 was no longer pushing temperatures up. The implication was that while there are both high and low values in the graph, the data no longer appears to have the seemingly dramatic increases that were apparent for the years from the 1970’s through the end of the century. And, with these few data points, it is likely that an average temperature line for the period could be reasonably represented by a flat line. This interpretation fit the presenter’s narrative that we were no longer experiencing global warming and were in for a shift to colder weather.
However, if we look at the same data for a longer period of time, we get a very different picture. The graph shown on the right shows the same series for 1880 to 2011. As you can see, temperatures have risen about six-tenths (.6) of a degree Celsius (1.08 F) between 1980 and 2011. Moreover, the continuing increase from 1980 through the first decade of 2000 is clearly obvious in this graph. And, there were many short periods that, taken by themselves, showed the same apparent halt in global warming. (see here).
The seemingly compelling nature of this comparison–that there is no evidence for the slowing of warming–is obvious in the second graph. However, you, as a reader or audience member, have to make up your own mind about whether a graphic presentation represents what is actually happening. This can be difficult for a layman without training in climatology (like me) as another example will make clear.
Below are two more graphs, this time going back 400,000 years. The data come from ice cores drilled in a part of the Antarctic that has been frozen over that entire period, see here and, more advanced, here. The blue line represents temperature and the red one parts-per-million of CO2. By the way, note that the CO2 never gets much above 290 parts per million for this- entire history prior to the present.
This graph seems to illustrate that atmospheric CO2 has little or no influence on overall climate shifts and has been essentially passive in the shifts of climate in the past. This is reinforced not only by the fact that atmospheric CO2 (the red line) seems to follow temperature shifts (the blue line), but that all the values of CO2 pretty much lie below, and seem to follow, those of temperature.
By contrast, if we change the scaling of CO2, to exclude today’s ahistorical high levels of CO2, we see a different picture. This chart, like the first one, shows that CO2 lags temperatures. However, this graph better illustrates the views of most climatologists: When global temperature and hence ocean temperature rises, CO2 is released from the ocean (in climatological speak “outgassed’) because warmer water can contain less CO2 than colder water. This reinforces the temperature rise, pushing it higher. And, when temperatures begin to decline, CO2 will be reabsorbed by the oceans. Indeed, from this visual perspective you can see a much closer tracking and interaction between temperatures and CO2. This is what most climate scientists have concluded happened.
The point here is that graphs, charts, and other visual media can be used to convey a message on a subliminal level, even one that is not supported by the science. So, when you see a graph or chart, don’t just assume it tells the whole story or that it accurately reflects the science. Good sites to get the latest and accurate climate science include Skeptical Science and Real Climate.
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