Thanks in large part to Al Gore’s book we’ve all seen the graphs showing temperature and CO2 levels for the last 600,000+ years (reproduced below). For many people today the image below serves as the iconic image of global warming. Al Gore even climbed onto a scissor lift to help make it so. Climatologists know that CO2 is a greenhouse gas; no one disputes this, and the graph underscores this by showing that, for the past 650,000 years, the peaks and valleys in temperature match up nicely with peaks and valleys in CO2. Mr. Gore then climbs onto the scissor lift, raises it up well above the screen displaying the graph and points out where CO2 levels are today, where they may be in 50 years and asks you to envision what the temperature will be like in 50 years.
The data in this graph comes from the study of ice cores in Antarctica and Greenland. Many books, when discussing ice core data, give a background of the science behind the data. We’re skipping that here. Instead we’ll accept the data as given and spend more time analyzing what the data means. But before we go on, can we accept the data as given? Despite the fact that the science behind ice core data is somewhat involved, none of the raw data is being questioned by anyone, skeptics and believers agree that the data in these charts are accurate. Many lay people wonder how it is possible to get temperature and CO2 readings for 600,000 years ago, but understanding how the data is collected is not necessary to understanding the lessons that the data is trying to teach, especially when there is no scientific dissent about this data. We can take it as given and move on.
Same Data, Different View
Now let’s return to Al Gore’s temperature and CO2 graph. This technique, of showing multiple pieces of data on the same graph is one of several that scientists use, especially if they have reason to believe that the data are correlated. It can highlight certain features and it is a very simple graph to understand as it is merely a combination of two even simpler graphs, a temperature vs year and a CO2 vs year graph.
Fortunately there are other ways to display data where we expect a correlation. Here, we’re expecting a correlation between CO2 levels and temperature. The graph above shows CO2 vs year and Temperature vs year, and we can see the correlation; the peaks and valleys in both graphs line up fairly well. But do we really care about the year? We accept that CO2 is a greenhouse gas and that large quantities of it in the atmosphere will cause temperatures to rise. But now that we’ve accepted that, aren’t we more interested in how high the temperature will rise for a given CO2 level? How strongly the CO2 and temperature are correlated with each other? One way to answer these questions is to remove the year from the graph and plot CO2 and temperature directly.
The graph above shows the 400,000 year ice core data plotted as we’ve been discussing. The first thing that stands out is that all the points do not fall on a neat little line. If variations in CO2 levels were the only thing that caused variations in temperatures then all the points would line up. But in reality there are many other things that cause temperature variations. Four hundred thousand years is a long time and the earth has seen a lot of changes in that time. Other greenhouse gasses besides CO2, methane for example, undoubtedly rose and fell during this time frame. The earth’s orbit changes significantly every 100,000 years, time enough for four cycles for the data shown above. And there were some large volcanic eruptions during this period, which put up clouds of material that have a cooling effect on the planet. And there are many other non-CO2 related causes that contribute to planetary temperature change. The point is that this graph correlates only CO2 levels with temperature and consequently we shouldn’t expect a neat line.
Let’s dig into this variation just a little more. For a given CO2 level these other factors could affect temperature strongly or weakly. The graph below highlights three temperature levels for the same CO2 level. Why aren’t these temperature values closer together? One reason is that these other factors could be strongly in play and making it either hotter or colder than would otherwise be the case. When the temperature is well above average, the earth could be closer to the sun and the methane levels could be high. When the temperature is well below average, the 100,000 year cycle in the earth’s orbit could put it farther from the sun, methane levels could be low and there could have been a large volcanic eruption. It is the variability due to the other factors besides CO2 that contribute to the variability of the temperature levels.
While the data doesn’t fall into a neat line, another thing that stands out is that this data does fall into a straight line. Recall, all we knew from the other graph was that low CO2 levels matched low temperatures and high CO2 levels matched high temperatures. We didn’t have good sense of how the values in the middle lined up. There are many ways the intermediate levels could have played out, but it turns out that a straight line is a fairly good approximation.
Where Are We Today?
This data covers the last 400,000 years, but it turns out the most recent data it contains is 2300 years ago. It begs the question “Where are we today?” To answer that we will need to re-draw the graph since the modern CO2 level is well outside the range of the graph above. The re-drawn version is shown below. The black bar at the bottom right shows the range of the modern CO2 level. The point above that bar is the approximate location for our climate today.
The data we’ve been discussing shows 3 things, the CO2 level is well outside the range of what’s been normal for the past 400,000 years. Somewhat more subtly, the temperature vs CO2 levels are also well outside the normal range. The CO2/Temperature point for today’s climate is no where near the imaginary line that passes through the middle of all the other data. At the current CO2 level our temperatures should be much higher. How much higher is more apparent on this graph than on Al Gore’s graph. Finally, only the temperature level is within the normal range. Our relatively low temperature today could be due to a natural lag in the response of temperature to CO2. It could be the case that the earth is already warming as fast as it can and more CO2 in the atmosphere won’t make it warm any faster. The current low temperatures could simple be because the temperatures haven’t caught up yet. It could also be that, as others claim, if it weren’t for the current CO2 levels we’d be in the early stages of an ice age. However, whatever the reason two implications are clear. 1) CO2 levels are crazy-high and we need to take steps to reduce them and 2) the current “low” temperatures aren’t a sign that global warming is false, but rather that we may have a bit of a grace period before temperatures start to rise. Let’s not blow it.
The ice core data for the past 650,000 years comes from 2 different sources.
From 420,000 years before present to now was published in:
Petit, J.R., J. Jouzel, D. Raynaud, et al 1999. Climate and atmospheric history of the past 420,000 years from theVostok ice core, Antarctica. Nature 399: 429-436.
and is available online here: http://cdiac.ornl.gov/trends/temp/vostok/jouz_tem.htm
From 650,000 years before present to 390,000 years before present was published in:
U. Siegenthaler, T. F. Stocker, E. Monnin, et al. Stable Carbon Cycle-Climate Relationship During the Late Pleistocene. Science, v. 310 , pp. 1313-1317, 25 November 2005.
and is available online here:
Information about the time lag between temperature change and CO2 change comes from several articles.
The 800 year lag is first presented in:
Caillon, N., Severinghaus, J. P., et al, “Timing of Atomspheric CO2 and Antarctic Temperature Changes Across Termination III.” Science, 14 March 2003, v299, pp 1728-1731
Another paper that suggests the correlation is even stronger than the data used in this article is presented in:
Cuffey K. M.; Vimeux, F. “Covariation of carbon dioxide and temperature from the Vostok ice corea after deuterium-excess correction.” Nature, 2 August 2001 v412 pp523-527