Despite dispute in the media, the scientific consensus is almost unanimous: the world is getting warmer as a result of human activity. But the scientific consensus is so clouded by politics that it’s very difficult for the average person to figure out what’s going on. I’m sick and tired of this, so what follows is a (hopefully) refreshing taste of dryness on the subject. No opinions, just the scientific consensus.
First off, some justification for the “almost unanimous” claim above. After analyzing 928 papers on climate change, one study confirmed that 75 percent explicitly or implicitly supported the notion of climate change as a result of human activity. The other 25 percent discussed methods or paleolithic changes in climate. Strikingly, none of them directly challenged the view that global warming was attributed to humans.
Of course, there are a wide variety of models and disputes about how we can expect global warming to progress, because the weather is a chaotic system, subject to the rules of complexity. Positive and negative feedback loops abound, creating a system that is very difficult to predict.
The best record we have of mainstream scientific conclusions on the subject comes from the Intergovernmental Panel On Climate Change, a collection of scientific data from the peer reviewed literature. The National Academy of Sciences has explicitly stated that its results are fair, and the American Meteorological Society, American Geophysical Union, and the American Association for the Advancement of Science have all issued similar statements.
I recommend taking a look at their report: Climate Change 2007: The Physical Science Basis. I’ll spend the most of the rest of this article explaining its conclusions.
Humans and Greenhouse Gas
- Using data collected from ice cores, scientists have concluded that the concentration of CO2 has risen from 280 parts per million in the pre-industrial age to 379 parts per million in 2005.
- The increase was larger between 1995 and 2005 than it was during any other time that it was directly measured (1.9 ppm each year).
- Methane, primarily from agriculture, increased from 715 ppb to 1732 ppb.
Humans and Warming
- The warming effect is measured by something termed “radiative forcing,” the net impact on how much energy is absorbed by the earth in comparison with how much is released.
- We can be 90 percent sure that the radiative forcing contribution from humans is between +0.6 and +2.4 Watts per square meter. That’s how many Watts are absorbed as heat instead of reflected into space as a result of human actions. This is in comparison with the year 1750, and it is explicitly a measure of the impact from humans, with all other known factors taken into account.
- Fluctuations in the sun have contributed a relatively small radiative forcing between +0.06 and +0.30 Watts per square meter.
- The biggest uncertainty in the contribution of humans comes from aerosols, which actually have a cooling effect between -0.1 and -0.9 Watts per square meter.
- The contributions from CO2, methane, and nitrous oxide are especially alarming, between +2.07 and +2.53 Watts per square meter.
- The linear warming trend over the past 50 years is nearly twice what it has been over the past 100 years.
- The temperature increase from the second half of the 1800s to the first five years of the 2000s is between 0.57 and 0.95 degrees Celsius.
- The ocean levels rose faster during the 20th century than during the 19th century. We can be 80 percent sure this effect wouldn’t happen if nothing changed.
- The total increase is between 0.12 and 0.22 meters.
Changes in Climate
- Since 1978, the arctic ice shrunk by 2.1 to 3.3 percent.
- We can be 66 percent sure that we experienced warmer cold days and warmer nights in the late 20th century as a result of human activity.
- We can also be 66 percent sure that hot nights were hotter than usual as a result of human activity.
- As for warm spells, heat waves, heavy precipitation events, droughts, cyclones, and extreme sea level incidents, we can be more than 50 percent sure that each of these increased as a result of human activity. (Bear in mind, this means there is less than a 3.1 percent chance that none of these increased as a result of human activity).
- There is insufficient evidence to suggest that the difference between daily highs and lows has changed. Atmospheric temperatures in Antarctica don’t appear to have changed, and there hasn’t been any statistically significant melting on the continent. There is no evidence yet that global warming has effected ocean currents or small scale phenomena like tornadoes, hail, lightening, or dust storms.
Comparison With Ancient Climates
- We can be 90 percent sure that average temperatures in the Northern Hemisphere were higher in the past 50 years than in any other 50 year period over the last 500 years.
- We can be 66 percent sure that they haven’t been this high in the last 1300 years.
- We can be 66 percent sure that, during the last interglacial period (125,000 years ago), sea levels were 4 to 6 meters higher than during the 20th century, and temperatures were 3 to 5 degrees Celsius higher, as a result of Earth’s position in orbit.
Attributing Climate Change
- We can be 90 percent sure that most of the increase in temperature is the result of greenhouse gases emitted by humans.
- We can be 66 percent sure that warming would have been higher if it weren’t for volcanic eruptions and human aerosols that created a cooling effect.
- There is only a five percent chance that the warming of the atmosphere and ocean, in combination with ice mass loss, would have occurred without external forcing.
- The lowest layer of the atmosphere has gotten warmer, and the next layer up has gotten cooler. We can be 90 percent sure this is the result of human greenhouse gasses and ozone depletion.
- We can be 66 percent sure that there has been significant warming on every continent except Antarctica as a result of human activity.
- Here’s a few graphs of continental temperature changes. Black represents actual figures. Blue represents 19 simulations from 5 models that only consider solar radiation and volcanoes. Red represents 58 simulations from 14 models that include human factors. The shaded regions contain 90 percent of the predicted possibilities.
- We can be 66 percent sure that human activity has had an impact on the wind. However, changes in North America have been larger than predicted by models.
- We can be 66 percent sure that extreme hot nights, cold nights, and cold days have increased as a result of human activity. It is more likely than not that heat waves have increased as a result of human actions.
- Climate sensitivity models predict that a doubling of carbon dioxide concentrations results in between 2 and 4.5 degrees Celsius, with 66 percent accuracy. There is only a 10 percent chance that this figure is below 1.5 degrees Celsius.
Projections for the Future
- If greenhouse concentrations were kept at year 2000 levels, warming over the next two decades would still be 0.2 degrees Celsius, due to the slow response of the oceans.
- We can be 90 percent sure that climate changes in the 21st century will be larger than in the 20th century if emissions stay where they are or increase.
- The expected temperatures will depend on how emissions change in the future. Constant 2000 levels would result in a temperature increase between 0.3 and 0.9 degrees Celsius by the end of the century. The suspected low emission scenario puts us in the 1.1 to 2.9 degree range. The suspected highest emission scenario puts us in the 2.4 to 6.4 degree range. (These figures are with 66 percent accuracy).
- The worst emission scenario puts sea level increase between 0.26 and 0.59 meters. The suspected low emission scenario puts it between 0.18 and 0.38 meters.
For an understanding of these scenarios: the assumed low emission scenario suggests that the population peaks in mid-century and starts declining afterward, coinciding with a rapid increase in IT and service jobs that reduce the need for raw materials, as well as the introduction of clean technologies.
The worst emission scenario assumes a similar population growth curve and rapid economic growth, but no dramatic shift toward IT and service and no serious change in fossil fuel use.
Now back to the data:
- Potential runaway effects caused by positive feedback loops between climate and the biosphere, or climate and the ice sheets, haven’t been included in these models because these phenomena are too complex.
- Ocean pH, a measure of acidity, is expected to decrease between 0.14 and 0.35 units over the next century (an increase in acidity).
- We can be 90 percent sure heat waves will increase.
- We can be 66 percent sure typhoons and hurricanes will increase.
- There is a 90 percent chance precipitation will increase in the high latitudes, and a 66 percent chance that most subtropical land regions will see a loss of precipitation.
- There is a 90 percent chance that oceanic currents will be affected, but less than a 10 percent chance that they will go through an abrupt transformation.
- To stabilize at 450 parts per million of carbon dioxide (compared to 379 ppm today), we would need to cut our emissions by about 27 percent.
Meaning for Humans
What does all of this mean for humans? Another IPCC report offers some conclusions:
- Global food production actually increases up to 3 degrees Celsius, and drops afterward. (Greater than 50 percent confidence)
- Above 2 degrees: malnutrition, diarrheal diseases, infectious diseases, floods, droughts, and extreme heat would create an aggregate increase in health problems (between 50 and 80 percent confidence), though this would be very sensitive to the effectiveness of the health system.
- With 80 percent confidence, hundreds of millions of people would face reduced water supplies above 2 degrees Celsius.
- With greater than 50 percent confidence, increases of 4 degrees or higher would adversely effect the economy and, as a result, most people in it.
I realize this article has been a bit dry in comparison to some of my other posts (and certainly most climate change posts), but that was intentional. Climate change discussions are so tainted with politics that I thought I’d pass along the least biased information I could for any who are as interested as I am in the truth.
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