Global Warming in the 2000s: Pit or Pendulum?by Bill Chameides | August 3rd, 2011
posted by Erica Rowell (Editor)
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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).
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?
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.
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:
- 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.)
- 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.
- 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
and: aerosols, air pollution, climate, climate science, climate skeptics, El Nino, El Nino-Southern Oscillation, ENSO, geo-engineering, geoengineering, greenhouse gas emissions, La Nina, Mount Pinatubo, particulates, solar cycle, solar minimum, solar radiation, sulfur dioxide, Sun, Susan Solomon, temperature records, volcanoes