Yesterday, thousands of people marched to bring attention to climate change worldwide. Here, I thought it would be useful to state the facts, simply and in one place. What is the evidence for human-induced climate change?
Yes. This is not an opinion; it’s a measurement. The surface temperature of the Earth has increased about 1 ˚Celsius (1.8 ˚Fahrenheit) since measurements first began in 1880. The year 2016 was the hottest year on record, breaking the previous record set in 2015, which broke the previous record set in 2014. In fact, 16 of the 17 warmest years have occurred since 2001. None of this is debated.
Are levels of carbon dioxide (CO2) in the atmosphere increasing?
Yes. This too is a measurement. In 2013, the amount of CO2 in the atmosphere rose above 400 ppm (parts per million), and it continues to increase each year. Although the amount of CO2 in the atmosphere fluctuates seasonally and over longer time frames, it has remained under 280 ppm for at least the last 10,000 years. This is not controversial.
Is CO2 a greenhouse gas?
Yes. This is based on basic laws of chemistry and physics. A greenhouse gas acts like panes of glass in a greenhouse. It lets in solar radiation and then traps heat re-emitted from the Earth’s surface. Without greenhouse gases, the Earth would be much colder than it is now. Adding greenhouse gases, such as CO2 and methane, to the atmosphere increases the amount of trapped heat and therefore warms the Earth. Some of the additional CO2 in the atmosphere is absorbed by oceans. There, it acidifies the water, which, along with increased ocean temperatures, is wreaking havoc on coral reefs and other marine life.
Is the increased CO2 the result of burning fossil fuels?
Yes. In part, we know this from the simple observation that we started burning significant amounts of fossil fuel at the same time that CO2 levels in the atmosphere began to rise, in the mid-1800s during the Industrial Revolution. However, this is a correlation, and correlation is not causation. Measurements of carbon isotopes implicate the burning of fossil fuels more directly. Isotopes are different forms of the same element with different numbers of neutrons. Most carbon atoms (99%) are 12C with 6 neutrons; about 1% are 13C with 7 neutrons; and an extremely rare form is 14C with 8 neutrons. Different sources of CO2 have different ratios of these isotopes. Scientists therefore measured the ratio of these isotopes in the atmosphere and found that the increased CO2 in the atmosphere comes from the burning of ancient organic matter (fossil fuels like coal, oil, and natural gas) and not from other sources such as volcanoes.
So, what’s uncertain and therefore open to debate?
We know that the Earth is getting warmer, the amount of atmospheric CO2 is increasing, and human activity is causing the increase. What we don’t fully know is what will happen to the climate over time and how different parts of the world will be affected. This is where models come in and all models come with a degree of uncertainty. Most of the climate models indicate that the Earth will warm 2-6 ˚C (4-12 ˚F) in this century.
So, the question is not whether the climate is changing or what is causing it. The questions are – To what extent will it change? What areas will be most drastically affected? And, most importantly, what can we do about it? These are the key questions that scientists and informed citizens, like the ones who took to the streets yesterday, are focused on.
© James Morris and Science Whys, 2017
Photo credit: Suzanne Siner