This is the third installment of a five-part series by Bill Chameides on How We Know Humans Cause Global Warming.
An argument I hear frequently from climate change skeptics goes like this: "Climate has undergone warming and cooling cycles for millennia. This is no different. It's just another naturally occurring warm cycle."
It's true that climate has undergone warming and cooling cycles for millennia, but it's not true that this is just another naturally occurring warm cycle. Here's why.
Scientists have made a study of the history of climate change (the field is called "paleoclimatology"), so we know quite a bit about past shifts in global temperature and what caused them. Carbon concentrations and temperatures can be tracked back hundreds of thousands of years by looking at ice cores, and millions of years using other proxies.
When using ice cores, CO2 concentrations are measured from air bubbles in the ice, and temperature is estimated from hydrogen isotopes in the water. (See my previous post in this series for an explanation of isotopes.)
The graph below, which is based on ice core data, shows CO2 concentrations and temperature from 400,000 years ago to the present. You can't see the 1°F to 3°F differences in global temperature that we are concerned with today because the temperature estimates shown in this graph are local for Antarctica, which has experienced less warming than the global average. Still, it shows that temperature has varied greatly over time, and this variation is strongly correlated with CO2 concentrations.
The major dips in the graph are ice ages, caused by shifts in the Earth's orbit around the sun. The warming from the change in how sunlight hits the Earth is amplified by an increase in CO2 concentration. The sequence goes like this:
- Orbital changes trigger an initial, relatively small temperature change on Earth.
- Melting permafrost and other changes caused by the warming release CO2 into the atmosphere.
- Since CO2 is a greenhouse gas, increased CO2 concentrations cause more global warming.
You also can see smaller ups and downs before modern times. These were caused primarily by variations in solar output and volcanic activity.
Now look at CO2 and temperature in the recent past – the far right of the graph. See how CO2 is rocketing up, and higher today than it's been in the last 400,000 years? Actually, CO2 concentrations are higher today than they've been in the last 650,000 years – this graph just doesn't go back that far.
Let's focus in on more recent history so we can get a better look. The graph below zooms in on just the last 1000 years, using temperatures for the whole Northern Hemisphere, not just Antarctica.
The natural phenomena that caused climate change in the past cannot account for what we see today. We're already in a warm period orbitally, as you can see from the first graph, and now suddenly CO2 concentrations and temperatures are skyrocketing.
This break in the pattern – even if you had no other information – strongly suggests something different is happening today. And as the other posts in this series show, something different definitely is happening. Today's abnormally high temperatures and CO2 concentrations come from the burning of fossil fuels. As you'll see in the last article in this series, nothing else can account for it.
Next up: the Medieval Warm Period. Stay tuned!