THEGREENGROK    Planetary Watch

The Ups and Downs of Global Haze

by Bill Chameides | August 19th, 2009
posted by Wendy Graber (Researcher)

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Haze (more severe in the left photo) not only obscures scenic views but also contributes to global dimming. (USDA-FS)

Tiny particles suspended in the atmosphere can affect the climate. How? A special issue in the Journal of Geophysical Research – Atmospheres sheds some light on the subject.

Variations in climate are affected by many factors:

  • Greenhouse gas warming is one, but not the only one.
  • The earth’s reflectivity or planetary albedo, today’s topic, is also of potential importance.

Global Dimming and Global Brightening

Clearly the more solar radiation that is incident on and absorbed by the surface of the earth, the warmer the earth will be. This does not require an actual change in the energy emitted by the sun–just changes in the properties of the atmosphere and the surface that determine the fate of the sun’s rays as they pass through the atmosphere and hit the earth’s surface.

If the atmospheric composition changes in a way that causes more of the sun’s rays to be reflected back to space before they reach the earth’s surface, the earth will tend to cool because less of the sun’s energy is absorbed by the planet. If the reflection is caused by the atmosphere (instead of the surface), we call that effect “global dimming” because the sun’s disk is obscured – although usually not so much as to be a visible dimming.

If the atmospheric composition changes to allow more of the sun’s rays to penetrate and reach the surface, the earth will warm. We call this “global brightening”.

Surface Solar Radiation

One way to track trends in global dimming and global brightening is through surface solar radiation (SSR) measurements, which have been recorded since the early part of the 20th century providing researchers with a rich monitoring record. The longest continuous record comes from Stockholm where monitoring data extend back to 1920. Long datasets also exist from remote locations. For example, continuous SSR measurements from the Arctic and Antarctic extend back to the 1950s.

Martin Wild of the Institute for Atmospheric and Climate Science, ETH Zurich has provided an excellent review of these datasets and placed them in a global context in the recent special issue of the Journal of Geophysical Research – Atmospheres dedicated to this topic.

The data from the diverse and disperse locations of the monitoring stations provide a remarkably consistent global picture. In the early part of the 20th century the data, while scanty, indicate a trend toward global brightening (favoring warming temperatures). During the mid-20th century, the trend reversed to global dimming (favoring cooling). Another reversal was seen from 1980s to the 2000s, a trend toward global brightening (favoring warming). There is some evidence that since the beginning of the 21st century there has been a leveling of SSR and thus a slowing or cessation of the global brightening trend.

Independent data on global dimming and brightening can also be inferred from satellite measurements and from “earthshine” –reflected light from the unlit side of the moon.  The reliability of the global dimming and brightening trends inferred from these sources is still being debated by the scientific community but appear to be consistent with the global trends extrapolated from the SSR monitoring stations.

What’s the Cause?

The data suggest that the ups and downs of SSR are being driven, to a significant extent by air pollution; specifically tiny particles suspended in the atmosphere, the same particles that cause hazy skies. They do this by causing photons that travel in one direction (say in the downward direction) to scatter into many directions including the upward direction.  The photons that are scattered upward can end up going right back into space. They are in other words reflected from the planet instead of being absorbed. And so, particles act to increase the reflectivity of the earth and thus cause global dimming.

Why the Trend?

The variations in SSR suggest that there was an increase in particle pollution in the mid-20th century followed by a decrease since the 1980s.  In fact global modeling calculations that simulate particle levels (primarily from the burning of coal) are able to reproduce the trend qualitatively – they get the trend direction right but underestimate the magnitude of the ups and downs.  The turnabout in the trend in the late 1970s and early 1980s appears to track the decreased reliance on coal and the adoption of emission controls in the western nations of the world.  Interestingly we have yet to see a large signal from the growing emissions from China, India, and the other rapidly growing economies.

What is the Climate Impact?

The input of energy from the sun obviously has a major impact on our climate and any change in SSR — any change in global dimming or brightening — must also affect global temperatures. And in fact, if one looks at the trends in global temperatures over the past 100 years, the influence of fluctuations in SSR is hard to miss.

  • The early 20th century was a period of fairly rapid warming, corresponding to a period of global brightening.
  • The mid-20th century saw little or no warming, maybe even some cooling, corresponding to a period of global dimming.
  • The end of the 20th century was another period of rapid warming and a period of global brightening.

Periods of global brightening and dimming (from M. Wild (2009) are superimposed on the GISS global mean temperature record (from NASA showing the correlation between dimming and cooling and brightening and warming trends.

Wow, quite a correlation. Given such a correlation, should we conclude that the global warming trend over the past 100 years is due to particle pollution instead of greenhouse gases? No.

First of all, scattering by atmospheric particles act to cool the planet; so they cannot be responsible for a net warming. So-called black carbon particles do cause warming, but even so they can not explain the net warming over the 20th century because the data suggest that there has been no net change in SSR over the period of the measurements. In other words, the ups and downs over the past 100 years have more or less canceled themselves out. Thus if global brightening and dimming were the only factor affecting global temperatures, the temperatures today would be about the same as 100 years ago. Clearly that is not the case.

The temperature record we see is a superposition of a number of effects acting at the same time. Some of the ups and downs we see in the record on multi-decadal timescales are very likely affected by particle pollution as described here. Variations on shorter time scales are likely related to the 11-year sun spot cycle and ENSO. And, with a high degree of certainty we can say that greenhouse gas pollution is responsible for much of the long term warming trend we see in the temperature record.

The Future

While not responsible for the net warming we have seen over the past 100 years, particles and their ability to affect global temperatures is one of the wild cards we must contend with in figuring out targets for future greenhouse emissions that avoid dangerous climate change. From a climate point-of-view one can think of particles as an ally – something to counteract greenhouse gas warming. But those same particles are one of the baddies of air pollution; they can be deadly to people. What’s a government to do?

Some have suggested a solution that includes particles but avoids air pollution: geo-engineering. They argue that we should inject particles in the stratosphere where they cool the climate without harming people. But that raises another whole host of issues.

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