How Computer Experiments Prove Climate Change is Caused by Us.

While there are still quite a few climate deniers out there, it seems the more savvy of them have switched tactics. It is not uncommon for former skeptics to admit that the climate is changing, however, they claim it is part of the Earth’s natural climatic changes. The problem with this argument is that it goes against all of our observations and some of the most fundamental laws of physics.

A crucial tool of science is repeated observation. Astronomers scan the skies for patterns, paleontologists comb the world for fossil specimens, physicists and chemists are lucky enough to be able to perform experiments over and over again. When I was a graduate student performing experiments on hydrogen, I worked hard to make sure I could keep under control as many factors as I could. I made sure the magnetic fields the atoms would encounter were the same for every experiment, I worked hard to ensure that the microwaves I was using were constant, and that the lasers were tuned up, so that every experiment was exactly the same as I could make it. If we couldn’t control something like gravity, we made sure that it wouldn’t have an effect on what we were doing. I would perform thousands of experiments a second, then I would change one thing, say the strength of the microwaves and perform thousands more experiments, then change the microwave strength again, and repeat. The whole enterprise rested on my ability to repeat the experiment over and over again, then make the changes I wanted to make.

With climate studies we are not so lucky. We have only one Earth to study. Climate scientists cannot do an experiment without any humans for millennia, then rewind history and repeat with humans, but no cacti. Climate scientists don’t even have the luxuries of astronomers, who with their powerful telescopes can scan the skies for years to find enough stars, or planets, or galaxies to sort into the categories that allow them to answer the questions they set out to do. They just can’t study the climates of more than one Earth, so why are scientists so sure that climate change is due to humans?

So we don’t have time machines and we don’t have a countless supply of Earths, but what we do have is computers. Really, really, really powerful computers. Computers and a great many research groups working on this problem from different angles. Oh, and lots and lots of experiments and observations.

These experiments are myriad. They include how greenhouse gasses interact with light, how light reflects from snow, how oceans interact with the atmosphere, how aerosols interact with clouds, etc. The results of all of these experiments are turned into computer code to predict the climate (see my article on the difference between climate and weather here). These computer programs divide the world into little interacting blocks where heat and matter flow from one to the other. By tracking the flow of energy from block to block, climate scientists create a model of the Earth’s climate. The problem is making sure that the computer programs are correct and that is where centuries of observations and measurements of greenhouse gas concentrations, ocean, and air temperatures are used. By comparing the results of their programs to these observations, climate modelers gain confidence in their understanding of all the drivers of climate change. As computers have gotten faster, scientists have been able to refine their models more and more. Currently the models can break the Earth up into cubes of roughly 18 miles on a side and take into account such subtleties as changing vegetation, the carbon cycle, atmospheric chemistry, as well as the basic energy flows in the atmosphere and oceans (IPCC Fifth Assessment, Working Group I).

The results of all of these different computer programs are compared to each other and the historical data and we find that the results are in good agreement with the long term historical trends of Earth’s climate. This gives scientists confidence in extending their models into the future to see how different responses of humanity to the threat of climate change can exacerbate or mitigate climate change of the future. Those predictions, however, will have to wait for another article. Instead, I want to show how these calculations can be used to show that current climate change is almost entirely due to us.

Fig. 1: The black curve shows the deviation of the global surface temperature relative to the average between 1961–1990. Adapted from IPCC AR5 WG1 FAQ (Figure 5.1)

The picture above shows the average deviation in the global surface temperature as referenced to the 1961–-1990 average. The negative anomaly for the years prior to 1940 show that those years were cooler than the 1961–1990 average, and the years after 1990 were significantly higher. For now we’ll ignore the red line. While we cannot do experiments on the Earth, reset them, and start again, we can do that in our computer models. We don’t need to include all of the effects in our code and that allows us to see which effects are the most important in driving modern climate change.

Fig. 2: How much the global surface temperature would change due to changes in the sun only. Adapted from IPCC AR5 WG1 FAQ (Figure 5.1)

In school you learned that the Earth orbits the sun not in a circle, but an ellipse. This is true, however, the Earth’s orbit is not as simple as that. The shape of the orbit changes ever-so-slightly over a period of a hundred thousand years or so. This means that over time the distance between the Earth and Sun changes, and when we are closer we get more energy from the sun and we expect the Earth to get a little warmer, and get colder when we are further away. We are also taught that the tilt of the Earth is what causes the seasons. Once again this is true, but the tilt of the Earth changes very slowly as well. Tilting back and forth over a time scale of 40,000 years. When the Earth is more tilted, we get more sunlight over more of the Earth, so the temperatures get warmer. There are a few other changes in the relative position and orientation of the Earth and the Sun, but you get the idea.

In addition to these orbital changes, the Sun changes as well. Currently the Sun is in its middle age, but it is brighter than ever, pouring out more energy than it did 100,000 years ago. Galileo pointed his telescope at the Sun and discovered it wasn’t as pristine as had been thought, but instead was covered in spots. We now know that these sunspots are cooler parts of the sun and that there is roughly a 11 year cycle of sunspot activity. This cycle corresponds to other cycles on the sun, which all contribute to the energy received by the Earth from the Sun to follow an 11 year cycle as well. These changes in the amount of energy we receive from the sun are seen in Figure 2 above.

Figure 2 shows the results of climate models that only include changes in the amount of energy we get from the sun. We observe the general 11 year cycle, but the overall trend in Figure 1 is not seen in Figure 2. Close inspection reveals that in the years after 1990 the affect of the solar variations was to lower the Earth’s temperature, in contradiction with Figure 1.

Fig. 3: Global surface temperature changes due to volcanoes. Adapted from IPCC AR5 WG1 FAQ (Figure 5.1)

Figure 3 shows the effects of only volcanoes to the global surface temperatures. Volcanoes have strongly affected the Earth’s climate in the past. This is because all of the aerosols the volcanoes put into the atmosphere act as a mirror of sorts. Aerosols in the atmosphere tend to reflect visible light from the sun back into space. If that light doesn’t reach the Earth, it can’t help heat it up. The thing about volcanoes and climate is that they only make the temperature go down. Volcanoes cannot cause the global temperatures to rise, furthermore the effects only last a few years or less. Again, we cannot match the black curve of Figure 1 by only including solar variations and the effects of volcanoes.

Fig. 4: Internal climate variability on global surface temperatures. Adapted from IPCC AR5 WG1 FAQ (Figure 5.1)

So the sun isn’t enough, nor volcanoes, but perhaps other climate scenarios can contribute? The answer is again, no. Figure 4 above shows that effects on global surface temperature due to changes in the climate due to normal fluctuations and to severe periodic effects like the El Niño-Southern Oscillation. The El Niño events are well known to cause the weather to get much hotter in a give year. We can certainly see very large increases in temperature in Figure 4 corresponding to El Niño years, but again we do not see the overall upward trend.

Fig. 5: Changes to global surface temperatures due to humans. Adapted from IPCC AR5 WG1 FAQ (Figure 5.1)

So we arrive at Figure 5 above. Figure 5 shows the calculated changes in global surface temperatures due to humans putting greenhouse gasses into the atmosphere, cutting down the rain forests, and putting aerosols into the atmosphere. Only here do we see the trend that reflects the temperature data. Here is Figure 1 again:

Fig. 1 redux: The black line is the observed global surface temperature. The red line is the sum of the effects in Figures 2–5.

The red line, which I ignored earlier is the sum of the contributions shown in Figures 2–5. Only when we include the effects of human activities do we get agreement with what we observe.

On April 22, 2017 scientists and citizens around the world marched. On April 29, 2017, the day that I write this thousands of scientists and citizens are marching again to call attention to climate change. For many, and for myself, it was to call attention to importance of decades spent in laboratories, the field, at a computer, at a chalkboard, arguing with colleagues to make sure that the best they had was going into these calculations. It is only with the satellites launched and maintained by NASA, the European Space Agency, and others that scientists have been able to get some of the data used to validate these computer models. It is only with the seemingly countless hours spent digging out ice cores from glaciers and the same number of countless hours spent analyzing the cores and figuring out what it all means that we can validate our models. The money and time spent on building faster computers and writing more efficient code gave us more confidence in our calculations. The miles traveled to go to conferences to debate and argue over the results and the techniques to make sure we hadn’t missed anything. The sleepless nights worrying that we didn’t screw up. This is what people were marching for. To remind people that it has been a big undertaking to understand our climate and it is an even bigger undertaking to figure out what to do now. We all need to be on board, because together we can.

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