Arguments
Aerosols have partially masked human-caused global warming
What the science says...
The global dimming trend reversed around 1990 - 15 years after the global warming trend began in the mid 1970's.
Climate Myth...
It's aerosols
Is a Thinning Haze Unveiling the Real Global Warming? (Kerr 2007) points out that the sunlight-reflecting haze that cools much of the planet seems to have thinned over the past decade or so. If real, the thinning would not explain away a century of global warming but it might explain the unexpectedly strong global warming of late, the accelerating loss of glacial ice and much of rising sea levels.
The global dimming trend reversed around 1990 - 15 years after the global warming trend began around 1975. So it can't explain what began the global warming trend. Aerosols have a cooling effect on Earth's climate. When aerosols thin, the result is a lack of cooling, not a warming effect. That's not just semantics - take aerosols out of the equation and in the absence of any other forcings, global temperatures would remain steady.
So what is driving the warming? In the past, solar variations have been the main driver in climate change. A comparison of solar activity and temperature over the past 1150 years shows a close correlation between solar activity and temperature. However, the correlation ends around 1980 when temperatures started rising but solar levels remained steady.
Another suspect in climate change is cosmic radiation which is thought to increase cloud cover (hence cooling the earth). However, again there has been no correlation between temperature and cosmic ray flux since 1970. In fact, all the usual suspects in natural climate change - volcanic activity, orbit wobbles, solar variations are conspicuous in their absence over the past 30 years of long term global warming.
The only forcing that causes warming and also correlates with current temperature rises is atmospheric CO2. It's risen 100 parts per million over the past 120 years - in the past, that kind of change has taken 5,000 to 20,000 years. As CO2 rose over the 20th century, the only mystery has been why global temperatures actually cooled from 1950 to 1980. I even read one study in 1980 where the researcher posed the question "why aren't we seeing any global warming with all this CO2 in the air?"
The answer is now apparent with recent studies in aerosol levels and global dimming. Atmospheric aerosols caused a global dimming (eg - less radiation reaching the earth) from 1950 to 1985. In the mid-80's, the trend reversed and radiation levels at the Earth's surface began to brighten. From 1950 to the mid-80's, the cooling effect from aerosols was masking the warming effect from CO2. When aerosol cooling ended, the current global warming trend began.
Intermediate rebuttal written by John Cook
Update August 2015:
Here is a related lecture-video from Denial101x - Making Sense of Climate Science Denial
This rebuttal was updated by Kyle Pressler in September 2021 to replace broken links. The updates are a result of our call for help published in May 2021.
Last updated on 4 August 2015 by MichaelK. View Archives
[DB] Note: This comment was moved from the Myth of the Mini Ice Age thread to this one, where it is much more on-topic.
[DB] Fixed bad html.
Does a higher atmospheric aerosol ( in the cloud condensation neuclii size range ) content result in a dryer atmoshere? Considering the particle size of dirty emissions between 1940 and 1970 and the cooling during that period, then the concerted effort to clean up emissions post 1970 coinciding with a rise in atmospheric water vapour content, should this correlation be considered as being part of the cause of the post 1970's warming or 1940 - 1970 cooling?
@ 7, first graph. To get a better picture of how changes in emissions between the 70's and more recent years would require a profile of the changing emissions particle size over that period. Below CCN size, smaller average particle size would result in longer residence time in the atmosphere, while they grow in size, raising the opportunity for the particles to bypass water vapour to higher altitudes leaving a higher WV content at lower altitudes. An examle of how raising emission standards can affect particle size can be seen in the following study " Measurement of Engine Exhaust Particle Size" by David B. Kittleson, ( pages 7 and 8 in particular ) where they describe the effect of new standards ( introduced in 1990 ) on deisel emissions.
carbtheory,
Most atmospheric aerosol is secondary in nature, meaning it is formed in the atmosphere through gas-to-particle conversion, not via direct emissions. Most direct emissions are coarse mode emissions of sizes >2-5µm, which have relatively short atmospheric lifetimes. Accumulation mode particles, which contain most CCN, have longer lifetimes, but are formed through gas-to-particle conversion, mostly involving sulfur dioxide (and resulting sulfuric acid) and ammonia. Thus, their abundance is largely driven by SO2 and NH3 emissions. The first is dominating the graph you refer to. SO2 emissions, dominated from coal combustion, have dropped since the 80s, but have risen again in the 00s due to coal combustion in East Asia.
The net effect of these aerosols is cooling because they increase Earth's albedo.
btw, CCN are not defined by size, but, as the name "Cloud Condensation Nuclei" says, they have the ability to take up a lot of water ultimately growing/leading to (cloud) droplets. Their existence (or increase) does not lead to drying in the atmosphere, but to a redistribution of cloud water content from larger to smaller droplets, also known as the Twomey effect, which makes clouds "brighter" (as seen from space), which has a cooling effect (search "ship tracks").
The emisisons standards induced changes most likely have only a small and regional/local effect. The biggest large-scale changes come from how much coal is burned and what end-of-the-pipe technology is used to clean the stack gas from SO2.
@ 19, first sentence, quite believable.
Second sentence: take a look at page 3 of the study I refered to, by mass the majority of emissions are between 0.1 um and 1 um. By number the majority are substantally smaller than 0.05 um for typicle deisel engine particle size. So, with an extremely high number weighting in what they describe as nueclii mode particles where does that leave us for potential CCN from those emissions once they start mixing with the atmosphere? This is for an internal combustion engine. However petroleum accounts for a higher % energy usage than coal.
Third sentence: "Accumulation mode particles", the name suggests they are growing in size, mass or both. Wikipedia state a 'typical' CCN size of 0.2 um. I'm under the impression that the accumulation mode particles that become CCN need to reach a minimum size which is dependant on how hydroscopic they are ( due to their chemical composition ) and the atmospheric conditions they exist in. Is this assumption correct?
carbtheory @20, first of all, you should not place all your argument on one publication, even if it turns out correct. That said, I presume you are talking about this presentation, given 14 years ago: http://www.me.umn.edu/centers/mel/reports/dbkucdavis.pdf
It shows on said page three (not including the front page), a "typical" diesel engine exhaust PM mass and number distribution. These (ultrafine and fine) particles are actually generated from gas-to-particle conversion, often inside the exhaust pipe. And yes, such traffic related emissions are responsible for a significant amount of fine PM in urban areas (not globally). The, most abundant in terms of numbers, ultrafine particles coalesce within hours to at most days into the accumulation range (the size range in the middle of that graph, labelled). They do not act as CCN themselves.
The accumulation mode is named such because that is where atmospheric PM "accumulates" after coalescence (i.e. from smaller particles) or deposition (i.e. removal of larger particles) over time. Due to steep number reductions (do not mistake emissions with ambient abundances), they do not significantly grow by coalescence any more, but mostly by condensation, i.e. more gas-to-particle conversion, which limits the overall growth rate and total size achievable. Their lifetime in the atmosphere is on the order of a week and thus they do not mix throughout the troposphere.
And yes, not all accumulation mode particles are good CCN. They have to be hygroscopic. Köhler theory describes what you are looking for. You can try this also.
The "energy usage" is not what matters in terms of how much PM is formed. Sulfur content is much more relevant. That is why it was mandatorily reduced in diesel a few years ago.