Yes, Human Activity Is Causing Climate Change

The evidence seems clear that the climate is changing, which makes the next question “what is the cause?” If you accept climate change as a reality, it doesn’t automatically follow that human activity is the cause. There should be evidence. Basically, that was the question my friend was asking yesterday morning, the question that I couldn’t answer for myself and went looking. The information below comes from the Intergovernmental Panel for Climate Change (www.ipcc.ch) website, particularly their 2013 report, from www.climate.nasa.gov, and from the website www.skepticalscience.com. The Skeptical Science website in particular was very clear and helpful although the IPCC report is more detailed.

Answering the question about what is causing climate change means talking about how the earth hangs on to heat. The earth is much, much warmer than the moon, even though we are approximately the same distance from the sun. Why is this the case? The difference is in our atmosphere. Besides being handy for us breathing animals, the atmosphere acts as a blanket. It traps heat from the sun and holds it close to the earth’s surface. Not all of the components of the atmosphere do this; the major gases that do are ozone, methane, nitrous oxide, and, of course carbon dioxide. These are the gases that are commonly called greenhouse gases, because they function similarly to the glass roof of a greenhouse in holding heat.

The major greenhouse gas that people contribute to the atmosphere is carbon dioxide. Carbon dioxide is produced chemically when carbon based fuels are consumed for energy. So, for example, when a human body breaks down glucose (sugar is carbon based energy, hence the name CARBOhydrate) on a cellular level to convert it to a usable form of energy oxygen is used up and carbon dioxide is produced, which is removed primarily through the lungs. When coal, oil and natural gas (also carbon based energy forms) are burned for energy carbon dioxide is also emitted.  

There is more carbon dioxide in the atmosphere than at any point in the last 800,000 years; currently (2014 data) about 397 parts per million, which is about 33% higher than the highest historical concentration of 300 parts per million. How do we know how much carbon dioxide there was in the atmosphere at various points over the last 800,000 years? The measurements are obtained from air bubbles trapped in polar ice cores. The uptick in the amount of carbon dioxide in the atmosphere over time correlates pretty well with the production of carbon dioxide through burning fossil fuels over time.

Public Domain Image. Source: climate.nasa.gov/key_indicators

Image Source: skepticalscience.com. Their Caption: Atmospheric CO2 levels (Green is Law Dome ice core, Blue is Mauna Loa, Hawaii) and Cumulative CO2 emissions (CDIAC). While atmospheric CO2 levels are usually expressed in parts per million, here they are displayed as the amount of CO2 residing in theatmosphere in gigatonnes. CO2 emissions includes fossil fuel emissions, cement production and emissions from gas flaring.

Correlation does not imply causation, of course, but in this case it is concerning, and there is not another explanation for the increase. The oceans are the biggest reservoir of carbon dioxide on the planet but the amount of carbon dioxide in the oceans (as measured by acidity; because carbon dioxide dissolved in water produces acid) is increasing, so the carbon dioxide isn’t coming from the oceans. Humans produce about 100 times the amount of carbon dioxide than volcanoes, so that’s not a good explanation either. Oxygen in the atmosphere is decreasing at about the same rate that carbon dioxide is increasing, which makes sense if the carbon dioxide is coming from burning fossil fuels (remember, converting a carbon based fuel to usable energy uses up oxygen. That’s why you need to breathe.) Finally, when you look at the particular isotopes of carbon dioxide in the atmosphere the signature of carbon dioxide (ratio if different types of carbon atoms) in the atmosphere are consistent with fossil fuel sources.

So, it looks like humans are generating a lot of carbon dioxide. How does that link to human activity causing climate change? There are a couple of indicators that it is. First, carbon dioxide absorbs extra heat at a particular frequency of infrared radiation. When infrared radiation away from the earth is measured, it is depleted in this particular frequency and more so the past 30 years. What this means is that over a recent period of time carbon dioxide, specifically, is holding more heat close to the earth’s surface. Second, the pattern of warming that we are seeing right now is that the lower part of the atmosphere is warming and the upper part is cooling; that is consistent with carbon dioxide trapping heat near the earth’s surface and preventing it from reaching the upper reaches of the atmosphere. It’s not consistent with an increase in solar radiation which would warm the entire atmosphere. Third, natural cycles of climate change don’t explain our recent warming.

Source: skepticalscience.com. Their caption: Annual global temperature change (thin light red) with 11 year moving average of temperature (thick dark red). Temperature from NASA GISS. Annual Total Solar Irradiance(thin light blue) with 11 year moving average of TSI (thick dark blue). TSI from 1880 to 1978 from Krivova et al 2007 (data). TSI from 1979 to 2009 from PMOD (see the PMOD index page for data updates).

Satellite measurements of the sun’s energy output from 1978 until now show a drop of energy output over the last 30 years, which would typically lead to a decrease in overall global temperature. The rate of warming is too fast to be explained by the earth coming out of the last ice age, and in fact, we should actually be slowly heading into a new ice age based on Milkanovitch cycles, which predict climate change as a function of variation in the earth’s orbit and tilt. Finally, if a natural cycle is causing the overall warming trend we are seeing, there also needs to be an explanation for why the increase in carbon dioxide that we can measure isn’t causing the warming. Centuries of chemistry and physics tell us that it should, so any alternative explanation has to account for this as well as explaining what is causing the warming. 

So, again based on the data I can find, it seems to me that human activity is responsible for the changes in the climate that we are seeing today. Which now leads me to my third question. Is climate change something to worry about?