6. Recent temperature rise is just natural variation. Or exaggerated.

Skeptics say the recent rise in observed global temperature is just due to natural variation. Or else the results are exaggerated, flawed or faked. So we don't have to worry about CO2 production.

While it is very clear that CO2 levels have increased, it is, after all, temperatures that actually drive weather. It has been widely reported that the "global temperature" has increased since 1975. However, the meaning and usefulness of the idea of a single global temperature has been questioned by skeptics and even some scientists.*

Fortunately, there have recently been very useful results for variations measured over time at the same location. The map above, recently released by NASA, shows average temperatures over the last decade at different locations around the world, relative to the 1951-1980 averages in the same locations.

As the map illustrates dramatically, warming was about twice as large in the Arctic compared to equatorial regions. The warming in the lower 48 of the United States was less intense than in many other areas, which may account for the false impression here that there hasn't been much warming lately.

We have elsewhere argued that the big controversy about whether or not at this point the Earth has warmed up due to human-caused factors is in reality just a distraction. We suspect that it is in fact a manufactured and deliberate misdirection by those desiring to confuse the issue, and to stall any effective response. There is, after all, no dispute that the CO2 levels have increased alarmingly since 1800. As was discussed in the last section, if CO2 levels haven't already caused an increase in temperature, the rising atmospheric levels certainly will eventually.

Temperatures relative to ice age variations
As mentioned in section 4, the climate during the last 1.3 million years has been locked into a cycle of repeated ice ages (glacials) separated by somewhat warmer "interglacial" periods. Judged from the standpoint of most of the Earth's history, however, we are currently in an ice age, even though we are in a so-called interglacial period. After all, we have year-round ice at both poles, permanent glaciers in many of the higher mountain ranges, and so on. We can say that we are in a long-term "ice age cycle."

Our conclusion is that any recent change in temperature should be judged against the background variation of the overall ice age cycle, not compared to earlier ages when the climate was much warmer.

Variations in temperature over time
In this section, we will look at restricted, but more complete, data sets to investigate changes in temperature with time. The lower panel in the figure above shows the derived variation in the surface sea temperature in the equatorial Pacific over the past 1.3 million years. Note the time axis varies in scale at points denoted by the vertical dashed lines. This is done to give detail to the most recent ice age, seen in the right third of the graph. The middle panel illustrates the range of temperatures and time scales generally seen in the ice age cycle. The left panel shows that there have been many "ice ages" during the most recent era.

The underlying cause of the ice age cycle appears to be a long-term "astronomical" forcing due to cyclical changes in Earth's orbit and inclination. This is not all of the story: There are feedback effects which tend to create stability in climate once ice has covered a large portion of the globe. These factors include the reflectivity of ice, and a decline in atmospheric CO2 levels which usually follows as a consequence of the start of the glacial period.

The upper panel expands the last thousand years, giving details of the variation of the temperature in the northern hemisphere. Before 1900, temperatures were estimated through "proxy" measurements of various kinds (light blue lines). After 1900, these were supplemented by direct measurements (red vertical lines). The heavy blue line is a smoothed version of the proxy data, using a 40 year moving average. The straight red dashed line indicates the long-term decline in the average temperature, a decline consistent with the known astronomical forcing.

Up until 1900, then, the trend was clear: We were headed for another major glaciation! Some idea of the conditions that would have occurred -- possibly sooner than later -- can be glimpsed by reading the history of the "little ice age" that affected most of the northern hemisphere around 1450 A.D.**

After 1900, however, something quite unusual occurred. The global temperature began to rise. Up to the period 1945-1975, during which there was a pause in the upward trend, the rise was not completely out of character with previous fluctuations typical of an ice age cycle. However, since then the temperature has shot up at a rate which is completely out of character for the ice age cycle that we had been in.

The publication of this graph in 2001 by the Intergovernmental Panel on Climate Change resulted in a sudden public appreciation for the possibilities of human-induced global warming. The upper panel of the figure above even obtained a certain fame (for a graph!) when it became known as the "hockey stick" graph. There has been considerable controversy as to whether this graph fairly included different historical proxy measurements or not.

However, it is clear that recent measurements have confirmed the trend shown. Although one may quibble at the exact numbers (and the skeptics do), the global temperature right now is probably higher than it has been at any time during the last 1.3 million years. If it isn't yet, it will be soon. Clearly we are headed into uncharted territory, and -- judging from the graph -- headed there at a very high speed.

Probable cause of the rise in global temperature since 1975

One question remains: Did the known increase in CO2 levels actually cause the sharp temperature increase observed since 1975? The difficulty is that there are many factors which can affect the global temperature, and all of these can vary and have varied with time. The next figure shows the most prominent factors: In addition to the levels of CO2 and other greenhouse gasses (blue line), there is the solar output (red line), the ozone level (green line), the sulfate level (magenta line) and the gases from volcanic eruptions (green line).

To estimate which component, in fact, is responsible for what change in global temperature requires a careful modeling of the climate. The figure indicates the relative contribution of each component which results in agreement of a computer calculation of the temperature of the model (gray line) with the measured temperature response (heavy black line). The amplitude of each component in the chart indicates how much that component affected the temperature, and in what direction.

The first thing to note about the model results is that they adequately reproduce the overall rise, including the pause in the period 1945-1975. It appears that.the reason for the pause was that warming effects due to increasing greenhouse gasses were canceled rather neatly by the blocking of light by the increase in (human-caused) sulfates in the upper atmosphere. Since that time, however, the model calculations indicate that the CO2 effect has overwhelmed the "benefit" from the sulfates. The results suggest that the CO2 levels have, almost entirely alone, been responsible for the steep rise in global temperature observed since 1975.

DETAILS

The map at top is from NASA, reported in a Science Daily article January 22, 2010.

The upper panel in the next graph is from the 2001 report by the Intergovernmental Panel on Climate Change The lower panel is from the article by Hansen, et al. (2006) "Global Temperature Change," Proc. Nat. Acad. Sci., 103, 14285-14293. Recent measurements have been schematically added to the lower panel.

Model results in the lower right graph are from Meehl et al. (2004) "Combinations of Natural and Anthropogenic Forcings in Twentieth-Century Climate," Journal of Climate 17: 3721-3727.

*See "Does a Global Temperature Exist?"

**Effects on agriculture and people during the period 1300-1850 -- the cold climate that we appear to be avoiding by artificially increasing the CO2 levels -- are detailed in an excellent book by Brian Fagen, "The Little Ice Age."

For details about the cause of temperature variations over the ice age cycle, see, e.g., the Wikipedia section on Milankovitch cycles. Likewise, a discussion of the "Hockey stick graph" can be found in the Wikipedia section, global warming controversy.

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