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Global warming might not be the only negative consequence of human made emissions of CO2 and other gases. 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 one of the main reasons for scientifical uncertainty about humans’ role in global climate changes. Aerosols like sulfur, nitrate, and organic carbon are formed in the atmosphere and cause global cooling. They sort of balance part 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.
The average climate effect from particles is a cooling effect. To which extent particles cool down the climate is yet to be answered by scientists. Important reason for this uncertainty is that estimates of the climate effect based on observations and models have not coincided. 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.
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.
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 between estimates from observations and models.
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.
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: that means that, if the production of particles should remain constant in the future, the amount of particles in the atmosphere would also be constant.
CO2 is different. it 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.