THEGREENGROK    Planetary Watch

Global Warming in the 2000s: Pit or Pendulum?

by Bill Chameides | August 3rd, 2011
posted by Erica Rowell (Editor)

Permalink | 5 comments


What happened to the climate over the past 10 years?

Over the last century or so the global temperature record is clear: Temperatures have risen. The increase in temps was especially pronounced in the 20-year period between 1980 and 2000 when global temperatures rose by about 0.7 degrees Fahrenheit (0.4 degrees Celsius).

Source: NASA’s Goddard Institute for Space Studies

But what happened in the 2000s? While the average temperature over the decade of the so-called aughts was the warmest on record, the annual temperatures over the period hardly changed at all. That this occurred while carbon dioxide (CO2) and other greenhouse gases continued to rise (see graph below) has led some to pronounce the death of global warming, and others to claim that CO2 and other greenhouse gases must not really affect climate. (See here and here.) Forget climate change, these folks argue — the whole thing has fallen into a pit never to be seen again.

I find such claims to be specious. Just look at the long-term temperature record. While the general trend is upward (almost certainly due in large part to greenhouse gas warming), there are plenty of multi-year intervals when the warming slowed or even halted. Consider for example the period from 1900 to 1920 and the early ’90s.

Why? Because greenhouse gases are not the only game in climate town. Other factors affect climate, such as:

It’s reasonable to assume that some combination of these other factors, superimposed on the long-term warming trend from greenhouse gases, can explain the shorter-term ups and downs.

However, quantitatively showing how this plays out in particular instances has been difficult because of a lack of data. But the 2000s are another story: We now have a far richer array of instruments (in space, afloat, at altitude, and on the ground) monitoring the Earth. What do these data say about what happened to temperatures in the 2000s?

Source: National Oceanic and Atmospheric Administration

Climate Scientists Put Their Models Where Their Hypotheses Are

In 2010 Susan Solomon of the National Oceanic and Atmospheric Administration et al reported that observed decreases in water vapor in the stratosphere may have provided some cooling over the decade. But not nearly enough to explain the whole decadal trend.

Now two recent papers — one on several additional cooling factors by Robert Kaufmann of Boston University and colleagues appearing in the Proceedings of the National Academy of Sciences (PNAS) and the other on “small volcanoes” by Solomon and colleagues appearing in the journal Science — have significantly added to the story.

Solomon et al: On the Role of ‘Small Volcanoes’

It has long been known that large volcanic eruptions can inject huge quantities of sulfur dioxide (SO2) into the stratosphere. Because the SO2 is converted into small particles that reflect sunlight back out to space, these eruptions cool the planet. (A very similar process is at the heart of the geoengineering proposal to offset greenhouse gas warming by injecting SO2 into the stratosphere.) The cooling effect of colossal volcanic eruptions has been well documented: for example, by a marked drop in global temperatures following the 1991 Mt. Pinatubo eruption.

Pinatubo and global temperatures. (From Hegrel et al (2006) and GISS

But with no major eruptions in the past 15 years, it has been assumed that volcanoes have not been affecting climate of late.

Not so fast, says Solomon and her colleagues. Using observations, the authors show that there has been a small but significant rise in the so-called background concentration of stratospheric particles. They argue that this rise is due to small volcanic eruptions such as: Soufrière Hills (Montserrat, 2006), Tavurvur (Papua New Guinea, 2006), Kasatochi (Alaska, 2008), and Sarychev (Russia, 2009). Using a climate model, Solomon et al show that this rise in stratospheric particles can provide a small but non-negligible cooling effect, canceling out about 20 percent of the warming expected from greenhouse gases.

But 20 percent is not 100 percent. Where does the rest come from? Kaufmann et al think they know.

Kaufmann et al: On the Role Played by Three Other Cooling Factors

Kaufmann and his colleagues identify three other cooling factors:

  1. Anthropogenic sulfur dioxide: Global coal consumption jumped in the 2000s — much of it from demand in China. This helped offset decreasing SO2 emissions from the United States and Europe with the adoptions of pollution controls. When coal is burned, SO
    2 emitted to the lower atmosphere is converted to particles that reflect sunlight and cool (similar to the stratospheric cooling described above). (An added wrinkle: A paper just published in PNAS by Joyce Penner of the University of Michigan and co-authors argues that the cooling effect of these types of particles are underestimated in most climate models.)
  2. Decreases in solar radiation: As the Sun goes through its 11-year sunspot cycle, solar radiation goes up and down, causing global temperatures to fluctuate a bit. After peaking in 2000, the Sun’s output declined as it headed toward an extended solar minimum that dominated much of the back half of the decade.
  3. El Niño-La Niña: The state of the southern Pacific Ocean moves between El Niño conditions (which increase warming, leading to relatively warm years) and La Niña conditions (which contribute to cooler years). (The phenomenon is often referred to as the El Niño-Southern Oscillation, or ENSO.) In 1998, a very strong El Niño occurred, amplifying warming, but since then, negative (La Niña) or neutral ENSO conditions have been more prevalent.

The authors find that including these cooling effects almost completely cancels out the warming effect of greenhouse gas increases over the decade of the 2000s. They conclude:

“The results of this analysis indicate that observed temperature after 1998 is consistent with the current understanding of the relationship among global surface temperature, internal variability, and radiative forcing, which includes anthropogenic factors that have well known warming and cooling effects.”

These are intriguing results. There is still much to be sorted out.

And lying quietly underneath these very science-y and dry studies is a worrisome implication. If correct, they would imply that the absence of a significant temperature increase in the 2000s was due to effects that are transient and easily reversed. If so, and they do reverse, Holy Toledo, we’d better watch out: The climate pendulum will be swinging fast in the warming direction.

filed under: carbon dioxide emissions, climate change, coal, El Nino, El Nino-Southern Oscillation, ENSO, faculty, global warming, La Nina, Planetary Watch, temperatures
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  1. Travis
    Aug 6, 2011

    That climate is impacted by a number of forces should be understood by all. However, in the American political or regulatory world, CO2 is pretty much the only game in town. Whether or not the supporters of AGW like, CO2 as the driver of AGW is still unproven. That will not deter EPA and the others who want to regulate our use of energy.

    • Bill Chameides
      Aug 15, 2011

      Travis: Actually the legislation that passed in the House and stalled in the Senate addressed non-CO2 greenhouse gases too. EPA’s annual greenhouse gas emissions inventory also includes more than just CO2.

  2. Pat Wentworth
    Aug 4, 2011

    The conclusions drawn here are wrong. One needs to examine the data – closely. The chart above shows a little more than .8ºC increase between 1900 and 2000. One then needs to look at how that information was collected. The chart states it is from “meteorological stations.” In 1900, everyone used a device called a “Stevenson Screen” – essentially a wooden shuttered box to house their instruments. Up until 1979 these boxes were whitewashed and sited away from buildings in full sun in a grassy area. With the just advent of latex paint, the boxes now yield a higher temp of .3ºF and a lower temp of .8ºF when compared to the whitewashed box. That alone makes one think a little. But with the introduction of computer-based recording instruments in the 80’s, situational bias became a very real factor. Electronic devices are now placed on rooftops, adjacent to air conditioner compressors or next to parking lots because they have to be hard-wired into the building’s computer system. Meteorologist Anthony Watts went around the country and used volunteers to document National Weather Service (NWS) recording stations and found that 89 percent of the stations failed to meet the NWS siting requirements that recording stations must be 30 meters (~100 feet) away from artificial heating or reflecting surfaces. ( With almost 9 out of 10 stations erroneously reporting higher temperatures, how can anyone say that we have “global warming” in the US. Worldwide the number of temperature-reporting stations drops each year. From a high of over 12,000 reporting stations worldwide in the late 60’s early 70’s to less than 6,000 stations today, how accurate can one be with “global warming” – or cooling. Show me the data. (

    • Bill Chameides
      Aug 15, 2011

      Pat: I do believe these issues have been addressed already. Moreover, how do you explain the temperature trend obtained from ocean-based measurements?

  3. Lou Grinzo
    Aug 3, 2011

    The part of this complex picture that we should find the most worrisome is anthropogenic SO2. As I understand the situation, China is also starting to clean up their sulfur emissions from coal plants, as part of an effort to improve air quality in some parts of that country. This has a huge potential to trigger the aerosol whiplash effect, what James Hansen calls our “Faustian aerosols bargain”. Right now, anthro. aerosols mask roughly 80% of the forcing from CO2 (latest IPCC report), so even a small decrease in their effect could mean an uncomfortably large percentage change in the effective forcing, and therefore warming, from everything we’re pouring into the atmosphere. The vast difference in the atmospheric lifetimes of SO2 and CO2 may be the most inconvenient fact of all.

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