In 2001, the IPCC (Intergovernmental Panel on Climate Change) concluded that “most of the observed warming over the last 50 years is likely to have been due to the increase in GHG concentrations”. But by 2007, the same organization upgraded that conclusion to:
“Most of the observed increase in global average temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic GHG concentrations.” (Source)
For those not up on the subject (and unwilling to use Google), “anthropogenic GHG” refers to human-caused greenhouse gases — which are gases in the atmosphere that absorb and emit radiation within the thermal infrared range. Common greenhouse gases in the Earth’s atmosphere are water vapor, carbon dioxide, methane, nitrous oxide, and ozone.
The report further notes:
“It is likely that increases in GHG concentrations alone would have caused more warming than observed because volcanic and anthropogenic aerosols have offset some warming that would otherwise have taken place.”
Scientists use words like “very likely” when something hasn’t been proven beyond a shadow a doubt, but when all or nearly all of currently available evidence points that direction. In other words, they’re hedging their bets, avoiding words like “certainly” or “without a doubt”.
(According to the IPCC’s Treatment of Uncertainty notation, “very likely” means greater than 90% probability.)
It’s politicians, lobbyists, and activists who change the “very likely” to “without a doubt”.
Why Do They Think So?
Ignoring for a moment the word-changing of the politicians, lobbyists, and activists, what exactly is it that’s caused huge numbers of scientists to conclude that humanity has “very likely” contributed to global warming?
There are several reasons, but the two I understood best are these:
* The warming properties of GHG are well-established
* Computer-based climate models cannot replicate the current observed warming, unless human GHG emissions are included
Scientists are well aware of natural forces that warm and cool the Earth’s surface, such as variations in the Earth’s orbit, volcanic activity, changes in solar radiation, and natural climate oscillations. All of these are factored into the computer models and studies.
So, they designed computer models that take into account all known factors, but leave out measurable emissions from human society, and run the program. It results in a cooler Earth than we currently have. When they include the known quantities of human-emitted GHGs, the computer model comes up with our current warming trend.
What We’ve Done
The human activities under consideration include more than just factories or cars spewing out GHG, and include global changes to land surface, such as large-scale deforestation and agriculture methods. Various reports (including this one, PDF) list the measurable totals of human-caused GHGs, and the sources.
77% Carbion dioxide
8% Nitrous oxide
24% Power production
18% Land use
Power Production: 24%
Not only is power production the largest source of these emissions, but it’s the fastest-growing source, partly due to developing countries and partly due to first-world countries increasing their demand for power. In the U.S. alone, electricity usage today is 13 times greater than it was 60 years ago, though our population has only doubled during that time.
Burning coal is the most common source of electicity production in the U.S.
Land Use: 18%
Almost all of this 18% comes from deforestation, which affects the climate in three ways: (1) reduces GHG-absorbing ability by removing trees which would normally abosrb carbon dioxide, (2) directly produces GHGs and aerosols during mass burning of trees, and (3) reduces reflectivity of the Earth’s surface.
Livestock production occupies 70% of the world’s agricultural land, or about 30% of the ice-free land surface of the Earth. Scientists name livestock as the source of 64% of all anthropogenic nitrous oxide emissions, 40% of all human-caused methane emissions, and 9% of all our carbon dioxide emissions.
(Livestock emissions are attributable to humans because the herds are grown specifically for us to eat.)
I first learned about this in 2006, from articles like this one (ABC News). It’s notable that mammals produce almost all of the livestock methane, and almost all of that is from cows, sheep and pigs (source, BBC).
If we cut our beef/lamb/pork consumption by half (each of us eat half as much meat), it would help the climate more than removing half the cars/buses/trucks from the world’s roads.
UPDATE, 2014.07.22: In a new study announced in 2014, researchers announced that cattle are far worse than all other livestock when it comes to greenhouse gases:
“Compared with the other animal proteins, beef produces five times more heat-trapping gases per calorie, puts out six times as much water-polluting nitrogen, takes 11 times more water for irrigation and uses 28 times the land…”
That’s a global figure of 14%. In the U.S., it’s higher: about 28% of the U.S.’s GHG emissions are from transportation (source, U.S. Dept. of Transportation). That figure doesn’t include the emissions from extracting and refining fuels or from manufacturing the vehicles.
When divided into types of vehicles, the U.S.’s GHG emissions come from:
34% passenger cars
28% light duty trucks (pickups, SUVs, minivans)
20% heavy duty trucks
7% commercial aircraft
3% ships and boats
2% other aircraft
For more, see the following entries, written mostly for myself, to help clarify my thought processes: