PopSci.com welcomes back Dr. Bill Chameides, dean of Duke’s Nicholas School of the Environment. Dr. Chameides blogs at The Green Grok to spark lively discussions about environmental science, keeping you in the know on what the scientific world is discovering and how it affects you – all in plain language and, hopefully, with a bit of fun. Now, PopSci.com partners with The Green Grok to bring you exclusive new blog posts a week before they hit the Grok's blog. Give it a read and get in on the discussion!
The changes in average global temperatures from 1850 to 2008 are illustrated below.
A warming trend since the early 1900s through today is clearly evident. The trend hasn’t been smooth and continuous, though; there were lots of blips and dips, and even an extended period, from the 1940s through the 1960s, without any warming (and perhaps even some cooling). But overall global temperatures have increased over the 100+ year period.
But what’s going on now? Is the globe still warming?
Even the Farmer’s Almanac appears to have gotten into the act.
Maybe here’s why?
Take a look at the graph below, which focuses on the last 20 years instead of the last 100. It’s easy to see why one might conclude that temperatures have been going down over the past 10 years. Do an experiment: Take your hand and cover up the graph to the left of 1998 – the year with the highest temperatures on record. It sure does look like temperatures have decreased over the past 10 years, doesn’t it?
But wait a second, why start with 1998? That’s kind of arbitrary. Suppose you covered up the graph to the left of 1996 (or 2000) instead of 1998 – a very different picture emerges. Similarly, if it were1992, and you looked back on temperatures in 1990 and 1991, you might have concluded that a cooling trend had started and you would have been wrong. So what’s going on?
Don’t confuse short-term temperature changes with climate change
It’s important not to confuse short-term or inter-annual temperature changes with longer-term changes in temperatures that are relevant to the issue of climate change, which occurs on decadal time scales. There are any number of factors that cause global temperatures to rise and fall from one year to the next. 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 – these are the two warmest years on record. That 1998 was so 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. Most scientists have concluded that 2005 was as warm as it was without the benefit of a solar maximum or strong El Nino because of extra warming from greenhouse gases, but that is an issue for another day.
Global warming from greenhouse gases does not occur in a vacuum; it occurs simultaneously with other factors that affect global temperatures, such as solar variations and El Nino/La Nina oscillations. 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, you have to “filter out” the short-term fluctuations. Scientists commonly do this by using multi-year averages of the temperatures. For example, the solid black lines in the first graph shown at the top of the post are 5-year running means, with each point on the line representing an average of that year’s temperature, that of two years preceding it, and the two following it. In a sense it is a smoothed-out picture of the temperature changes with the year-to-year extremes averaged out.
So is the global climate on a cooling trend?
To answer that question, I segmented the temperature record into 5-year groupings starting from 2004–2008 and going backward, then calculated the average global temperature for each 5-year segment. The results are illustrated below by the dashed lines.
Through this lens, we see evidence of a slowing of the warming trend but not of cooling. The past five years (2004–2008) were on average warmer than the previous five years (1999–2003) , which were warmer than the previous 5 years, and so on down the series. However, the difference between the 2004–2008 average and the 1999-2003 averages is not statistically significant. No cooling there. OK, but some of you are probably still wondering why 2008 was such a cold year. Two things:
1. The stars (so to speak) were apparently aligned to make 2008 a cold year. The Pacific Ocean was in a La Nina phase, which favors cold global temperatures, and the sun was going through (and by the way continues to go thorough) an unusually strong minimum in its 11-year cycle; and
2. Even so, 2008 was not that cold. 2008 was actually the eighth or ninth warmest year on record. How could 2008 be the eighth warmest despite La Nina and the solar cycle? Greenhouse gas warming anyone?
So, what about those pronouncements of a global cooling? I would have to categorize them as – how can I put this – myopic?
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