I have a friend, we’ll call him Joe, who doubts whether climate change, is real. He argues that we’ve only been measuring temperatures for about 150 years, and that is not long enough to detect or predict climatic changes. He’s right; we couldn't say with certainty that the world is warming based on 150 years of data. But thanks to the science of paleoclimatology, the study of ancient climates, we don't need a thermometer to get a pretty accurate picture of the global temperatures and climatic trends of long ago.
One way to evaluate past temperature trends is to look at the geothermal gradient below the earth's surface. As you descend through the earth's crust, temperatures increase at a predictable rate. The only thing that affects the rate of increase is changes in surface temperature, and those deviations provide an accurate historical temperature record. Tens of thousands of boreholes have been drilled worldwide, and analysis of the temperature data shows that the world is warmer now than it has been for the past 500 years.
So that takes us back 500 years. I suspect Joe would argue that’s still not long enough to definitively say our climate is undergoing a significant warming. So can we look farther back into the past to see what the climate was like? We can, using what scientists call proxies.
Simply put, a proxy is something that substitutes for directly measuring temperatures. It provides evidence of climatic conditions at a given point in time. There are quite a few different proxies used to construct past climatic histories. Here are just a few:
What is most compelling about using these and other proxies to detect past climatic conditions is that they are all in agreement. The world is warmer now than at any time in at least the last 100,000 years.
This is further supported by the many indications we see today of rapidly changing climatic conditions, including the significant northward shift of many plants and animals, the smallest arctic sea ice extent ever recorded and sea levels rising 80 percent faster than was predicted in the worst case scenario just a couple of years ago.
I wonder if this will be enough to convince Joe?