Global Warming and Predictions of an Impending Ice Age: Global Warming Since 1998

This is the third post in a 4-part series on the connection between the sun, sunspots, and climate.

You don’t have to search too hard to find a skeptic's blog proclaiming that global warming “stopped” in 1998. Oh happy day if it were true, but sadly it is not. Why do I say this? I’ve looked at the data.

Take a look at the graphic below, which shows the average global temperatures from 1990 to the present. The green diamonds show the 5-year averages for the periods from 1988–1992, 1993–1997, 1998–2002, and 2003–2007. Each successive diamond appears at a higher temperature than the one before. In other words, global temperatures have been increasing over the past 15+ years -- global warming has not stopped.

Global temperature trends since 1990. Solid line with small dots indicate the annual averages. The green diamonds indicate the 5-year averages. Data taken from the Carbon Dioxide Information Analysis Center at the Oak Ridge National Laboratory: http://cdiac.ornl.gov/climate/temp/temp_table.html

If you look at the temperatures in the graphic year by year, it’s easy to see why someone might think that the warming has stopped. After all, there was a huge temperature maximum in 1998. Since then, only 2005 had average temperatures equal to or perhaps slightly greater than those in 1998.

Eyeballing temperatures from 1998 onward might lead to the inference that temperatures have not increased at all -– that global warming has stopped. But wait. If you do the same eyeballing exercise starting in 1999 or 1996 you would conclude that there has been a rapid increase in temperatures. Moreover, if you were back in 1992 or 1993 and had done the same eyeballing exercise back to 1990, you would have concluded that global warming had stopped; and you would have been wrong.

So what’s the problem? It comes from a confusion between inter-annual and short-term temperature changes and the longer-term changes in temperatures that are relevant to the issue of climate change on decadal time scales.

There are any number of factors that cause global temperatures to rise and fall. Solar activity is one –- as the sun goes through its 11-year sunspot cycle, solar radiation goes up and down causing global temperatures to fluctuate up and down. El Nino and La Nina oscillations in the South Pacific Ocean also lead to relatively warm years (El Nino) and cool years (La Nina).

The years 1998 and 2005 are interesting to compare. Depending upon the method used to analyze the temperature data, scientists have concluded that either both years tied for the warmest temperatures on record or 2005 was slightly warmer (see here or here).

That 1998 was unusually warm is not surprising. It was a year with an unusually strong El Nino and with the sun close to its 11-year maximum. By comparison, the sun in 2005 was near the minimum in its cycle, and the year began with a weak El Nino that dissipated by late spring. A reasonable explanation for 2005 being as warm or warmer than 1998 without the benefit of a solar maximum or strong El Nino includes warming from greenhouse gases.

Global warming from greenhouse gases does not occur in a vacuum; it occurs simultaneously with other factors that affect global temperatures like solar variations and El Nino/La Nina oscillations. As I discussed in my previous posts in this series, these other factors can cause short-term ups and downs in global temperatures. But the question for global warming is whether they cause a net temperature change. To determine that, we filter out the short-term fluctuations by using longer term averages (such as the 5-year averages shown in the graphic), and when we do, the upward trend in global temperatures comes through loud and clear –- take a look at the green diamonds.

In my next and final post in this series, I’ll take a look into the crystal ball and examine where climate might go in the coming decades.

Other Posts in Global Warming and Predictions of an Impending Ice Age

Part 1: Total Solar Irradiance

Part 2: Sunspots

Part 4: Predicting Future Climate