The mystery of particles

There is large scientific agreement that human made emissions of CO₂ and other gasses give global warming. But human activity doesn’t just cause gas emissions. Burning of fossil fuels and biomass also causes emissions of the particle black carbon. Other kinds of particles are formed in the atmosphere as a cause of human made emissions.

Particles, also named aerosols, are today one of the main reasons for the uncertainty about how humans affect the global climate. Aerosols like sulfur, nitrate, and organic carbon are formed in the atmosphere and cause global cooling. Thereby they contribute to mask parts of the human induced global warming. On the other hand, black carbon absorbs radiation and thereby has a warming effect on the earth’s climate.

Models and observations

The average climate effect from particles is a cooling effect. But to which extent particles cool down the climate, has remained an unanswered question for scientists.

“An important reason for this uncertainty is that estimates of the climate effect based on observations and models have not coincided”, Gunnar Myhre explained.

“For scientists, this has been frustrating. It has given us less faith in our understanding of the models and in our understanding of the direct aerosol effect”.

Defect in the estimates

Based on satellite observations, estimates are made of the climate effect from aerosols. If these estimates were correct, aerosols would have had a stronger cooling effect than the models show and thereby to an even larger extent have masked the global warming. Some estimates even show that aerosols have masked as much as 50 percent of the warming from CO₂.   

But Myhre’s article now points at what might have been a defect in these estimates. By doing this, he brings scientists a big step closer to the explanation of the discrepancy between models and estimates from observations.

“The estimates are not able to consider that the share of black carbon particles has increased by a much faster rate than the total number of particles. This can explain the main discrepancy we have seen between estimates from observations and models”, Myhre said.

Weaker cooling effect

According to Myhre’s article, the models have until now given the best picture of the climate effect from aerosols. The cooling effect from aerosols looks like being a bit weaker than the estimates from observations would say.

“What I have done, is a small contribution to our understanding of the human influence on climate change. The more we understand, the better prognoses can we give for future temperatures”, Myhre said.

Even though particles until now have masked part of the global warming, they will not do this to the same extent in the future.

”Particles only stay in the atmosphere for a few days. If the production of particles should remain constant in the future, the amount of particles in the atmosphere would also be constant”, Myhre said.

”CO₂ is different. CO₂ stays in the atmosphere for several hundred years. With constant CO₂ emissions, the concentration continues to increase and the warming will accelerate strongly in the future. The aereosols will then be able to mask a relatively smaller part of the global warming”, Gunnar Myhre said. 

More information: Consistency Between Satellite-Derived and Modeled Estimates of the Direct Aerosol Effect
Gunnar Myhre, Published online June 18 2009; 10.1126/science.1174461 (Science Express Reports)

Source: Centre for International Climate and Environmental Research