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Sustainability Matters 20: Anthropogenic Climate Change


Anthropogenic Climate Change – The Human Connection

Up to this point, we have discussed temperature and carbon dioxide and shown that increases in the amount of carbon dioxide in the atmosphere closely correlate over long periods of time with rises in the average planetary temperature.  

We have observed that temperatures on the earth are warming, and measurements of CO2 in the atmosphere are their highest in millennia.  What are the mechanics of this process which causes carbon in the atmosphere to hold heat in?  How do we know that human activity is the source of this dramatic increase in carbon?  

CO2 is the predominant constituent of the chemicals known as greenhouse gasses.

When sunlight hits the atmosphere, about ⅔ of the visible light (short wave radiation) that strikes the top of the atmosphere makes its way through to the earth’s surface and warms the planet. The earth radiates some of this heat back into the atmosphere in the form of long wave, infra-red radiation. Greenhouse gasses absorb as much as 90% of this infra-red radiation, most of which would otherwise dissipate into space. As a result, these gasses hold heat in the atmosphere, like glass holds heat in a greenhouse. 

The following tables from the Center for Climate and Energy Solutions list the major chemicals which contribute to the greenhouse effect, their human (anthropogenic) sources, how long they stay in the atmosphere, and their Global Warming Potential (GWP).  The second chart compares the amount of each of these gasses in the atmosphere in 1750 (before the Industrial Revolution) with the amount found today.  



Chemical Formula



Atmospheric Lifetime1(yrs.)

GWP2(100 Yr. Time Horizon)




Fossil-fuel combustion, Land-use conversion, Cement Production





Fossil fuels,

Rice paddies,

Waste dumps







Industrial processes, Combustion



Tropospheric Ozone 


Fossil fuel combustion, Industrial emissions, Chemical solvents





Liquid coolants,









Sulfur Hexafluoride


Dielectric fluid





Pre-1750 Tropospheric


(parts per billion)

Current Tropospheric


(parts per billion) 







1,870 / 1,7488




323 / 3228

Tropospheric Ozone 





.534 / .5328



.218 / .19410

Sulfur Hexafluoride


.00712 /.006738, 10

Figure 3-40:   Greenhouse gasses in the earth’s atmosphere

Notice that CO2 has the lowest GWP potential of these gasses, but because of sheer volume put into the atmosphere, carbon dioxide remains the dominant greenhouse gas.  On the other hand, methane, which is emitted when natural gas is burned, has a GWP 25 times greater than CO2 The fact that methane is such a potent greenhouse gas has led some to say that switching from coal and oil to natural gas is like an alcoholic replacing whiskey with beer. 

We can get an idea of the effect of these gases on earth’s environment by comparing the earth with the moon and the closest planets, Venus and Mars.

Venus, slightly closer to the sun and a bit smaller, has a runaway greenhouse effect. The atmosphere is 96% CO2, with an atmospheric pressure 91 times that of the earth and an average temperature of 867 degrees F.  

The moon, with no atmosphere or atmospheric pressure, has an average temperature of -4 degrees F. Mars has an average temperature of -85 degrees F with its CO2 and H2O frozen solid. The CO2 atmosphere on Venus has caused a runaway greenhouse effect and scorching temperatures. The lack of an atmosphere on the Moon and Mars leaves them both in a deep freeze. The Earth by contrast has an average temperature of 59 degrees F and an atmosphere of nitrogen, oxygen, water and carbon dioxide. 

Though no one is suggesting a Venus-like runaway greenhouse effect will occur on the Earth, Venus shows us in the extreme, the connection between CO2 and temperature.  

We understand the physical process which occurs when CO2 and other greenhouse gasses trap heat in the atmosphere. We’ve measured the increasing amounts of carbon in the atmosphere. However, that remains us with one more key question.  

How do we know that the increased CO2 is from human activity?   There are several ways we know this.

The first is through accounting. We can correlate historical records of fossil fuel use which track how much carbon humans have put into the atmosphere. 

Other evidence can be found by sampling the atmosphere. 16C, which is found in all living material, has a half-life of about 5,000 years. Since fossil fuels are millions of years old, any 16C which was present in the fossil fuels consumed by humans has long since decayed. Therefore, by sampling the atmosphere and determining the ratio of carbon isotopes, we can not only measure an increase in the amount of carbon in the atmosphere, as the above graph shows, but also determine its source. That source is fossil fuels burned by humans.   

The following chart chronicles the increases in anthropogenic greenhouse gases over a 40-year period. 

Figure 3-41 Anthropogenic Greenhouse gas emissions 1970-2010 

On the subject of the causes of climate change, each of the five IPCC reports has stated in successively stronger terms that climate change is anthropogenic – that is, it is caused by humans.  This is what the most recent report had to say on the subject.

Anthropogenic greenhouse gas emissions have increased since the pre-industrial era, driven
largely by economic and population growth, and are now higher than ever. This has led to
atmospheric concentrations of carbon dioxide, methane and nitrous oxide that are
unprecedented in at least the last 800,000 years. Their effects, together with those of other
anthropogenic drivers, have been detected throughout the climate system and are
extremely likely to have been the dominant cause of the observed warming since the
mid-20th century.
{1.2, 1.3.1} 

What are some Future Scenarios, Computer Models and What can their models tell us?

One of the many tasks performed by the IPCC was to see how accurately models would reflect data we already had as a test of their accuracy prior to making predictions of the future. A computer model is a mathematical representation of reality.  While we can’t test today a model’s actual accuracy at predicting what will happen 50 years from now, models can be tested in much the same way we’ve seen proxy data validated.   By comparing computer simulations with actual observed data, computer models can be assessed for accuracy at predicting known conditions. This process also helps to calibrate a model, improving the model’s level of accuracy. 

We have climate records including CO2 and temperature data dating back to pre-industrial times. Different teams of climate modelers were given the same task – make an accurate model starting with the known conditions prior to the Industrial Revolution.  Each group developed and tested their models independently of the others. The only models that could predict current conditions were those which included the CO2 generated by human activity. Though each team’s model had different inner workings, each of the models came up with similar predictions to explain current conditions.  

The value of this process is twofold. First, the results are one more piece of evidence linking CO2 emissions and changes in the climate. Second, and similar to our discussion of proxies, these results serve as calibration, but instead of just looking into the past as our proxy data does, computer models can look into the future.  

The IPCC modelers explored the outcomes of 300 baseline and 900 mitigation scenarios on climate conditions in the year 2100. A scenario means a set of input conditions related to the “main driving forces of future GHG [greenhouse gas] emissions… demographic change, social and economic development, and the rate and direction of technological change” to predict the most likely outcomes for each of these scenarios. The scenarios ranged from maintaining our current energy use habits (“business as usual”) to a future path where humans switched to predominantly non-fossil fuel/renewable-based energy sources. 

The following graphs show the range of outcomes of these different scenarios. The colored areas represent the paths of various scenarios, categorized by the amount of CO2 they predict in the atmosphere in the year 2100.  

The solid lines represent four of the 100’s of scenarios which were run. Those four scenarios include a stringent mitigation scenario (RCP 2.6) which significantly lowers greenhouse gas emissions, two intermediate scenarios (RCP 4.5 and 6.0), and one scenario with very high GHG emissions (RCP 8.5) which represents “business as usual”.

Figure 3-42: IPCC Climate Change 2014 Synthesis Report page 9

The following graph describes the relationship between cumulative carbon emitted, measured in parts per million (ppm), and temperature in the year 2100.  It once again shows a direct, linear relationship between cumulative CO2 in the atmosphere and temperature rise. 

Figure 3-43: IPCC Climate Change 2014 Synthesis Report page 9 

Global mean surface temperature increases at the time global CO2 emissions reach a given net cumulative total, plotted as a function of that total, from various lines of evidence. Colored plume shows the spread of past and future projections from a hierarchy of climate- carbon cycle models driven by historical emissions and the four RCPs over all times out to 2100, and fades with the decreasing number of available models. Ellipses show total anthropogenic warming in 2100 versus cumulative CO2 emissions from 1870 to 2100 from a simple climate model (median climate response) under the scenario categories used in WGIII. The width of the ellipses in terms of temperature is caused by the impact of different scenarios for non-CO2 climate drivers. The filled black ellipse shows observed emissions to 2005 and observed temperatures in the decade 2000–2009 with associated uncertainties.

The two graphs above help us to visualize the key findings of the IPCC. They show the relationship between how much CO2 humans put into the atmosphere and the amount of temperature rise predicted to occur between 1861 and 2100.  If we carry on as usual, we can expect in the range of a 5 degree C rise in temperature, and the serious socio-economic problems which will accompany such changes.

The models show that we can still limit the rise in temperature to 2 degrees Celsius by keeping concentrations of CO2 to the 450-500 ppm range.  This would significantly limit the damages and effects of climate change on humans relative to the “business as usual” scenarios.

Emissions scenarios leading to CO2 equivalent concentrations in 2100 of about 450 ppm or lower are likely to maintain warming below 2°Cover the 21st century relative to pre-industrial levels15. These scenarios are characterized by 40 to 70% global anthropogenic GHG emissions reductions by 2050 compared to 2010 16 

The scenarios which successfully limit temperature rise to 2 degrees C over pre-industrial levels achieve that goal with a reduction of 80% or more in CO2 emissions from electricity generation by 2030. To accomplish this effectively requires the decarbonization of electricity generation.  We must replace our current generating capacity with renewable energy or we, and our descendants, will face severe consequences. 

We are in a unique place in history. Questions about the place of science in policy making abound. How should we use the information we have learned?  

I am here in my kitchen, all the windows closed, no breeze, holding a new, unfolded 8½” x11” piece of paper in the air in a horizontal position six feet above the ground.  I propose we vote on what will happen when I let the paper go. Does anyone really think it will float up to the ceiling? (Even if I pump millions of dollars into ads promoting the idea?)  The idea is ridiculous and we all know it. There is an objective reality that no amount of wishing or voting for will change.

We know the paper will ultimately land on the floor, but its exact path is much harder, if not impossible, for us to predict. However, we don’t need to know its exact path to understand the ultimate outcome – the paper will land somewhere on the floor.  

We have explored how we know that our climate is changing, and how we know the cause is anthropogenic.  Like the paper being dropped on the floor, we know that the climate will continue to warm, but not the exact path it will take. Unlike the paper being dropped, the conclusions of the IPCC suggest we still have a good chance of limiting this warming and its negative effects on humans with appropriate action. 

Scientific research has established the facts quite conclusively that the planet is getting warmer and that human actions are a large part of the cause.  What we do with that knowledge is up to us. 

In our earlier discussion of the scientific method, we stated that there is no automatic cause and effect between what we know and how we choose to act. That decision is up to us and represents the greatest challenge humans have yet faced.  

What did you learn from this week’s post?

Photo by Wes Golomb

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