Often, the argument boils down to two “oh yeah” positions: “Anyone can see it’s getting warmer” and “We’ve always had hot years; this is no different.”
So who’s right? Is there any evidence pointing one way or the other?
It turns out there is.
Since the mid-nineteenth century, air and sea temperatures have been gathered from weather stations around the world. The data are then analyzed, scrutinized to eliminate artificial effects (such as the “heat sink” factor from urban areas), and combined to produce an average global temperature that is then compared with some baseline average.
For example, NASA’s temperature chart uses a 30-year average from 1951 to 1980, estimated at 57.2 degrees Fahrenheit.
The temperature chart reveals some remarkable worldwide trends:
• The 20 coldest years on record all occurred before 1919.
• The 20 warmest years have all occurred since 1988.
• The last time a record was set for the coldest year was 1890.
• The warmest-year record has been broken 19 times (with 2010 the hottest year globally).
• The warmest-year record was broken 12 times between 1881 and 1987, yet none of those years even make the top-20 list, not even 1987, the warmest year in more than 100 years of record keeping.
• Between 1880 and 1936, every year was below the 1951-1980 average.
• Since 1977, every year has been above the average.
• The first time the average global temperature reached 58 degrees F was 1998; since 2001, every year has averaged more than 58 degrees F.
These trends indicate that over the past 132 years the Earth has been warming at an ever-increasing rate with an overall rise of about 1.3 degrees F.
If yearly temperature fluctuation were random, we would not expect to see the pattern that emerges from the data. But what if we look at a longer timescale? Couldn’t this trend be a natural fluctuation that has been repeated many times in the past and will naturally correct itself? After all, global temperature data go back only to the 19th century. If the data went back further, might we not see similar warming trends that eventually cooled off?
It turns out we do have other data that let us look back further in time.
For example, tree rings in temperate zones, which vary annually according to moisture, temperature and the length of the growing season, contain a year-by-year record going back 1,000 years.
Analysis of these rings indicates that we are warmer now than anytime within the last millennium.
In tropical areas, coral reefs, which form annual temperature-sensitive layers of calcium carbonate, reveal a similar pattern.
And in polar regions, annual temperature data can be inferred from ice cores through analysis of the ratio of two oxygen isotopes. (Isotopes are atoms with the same number of protons but a different number of neutrons.)
Once again, the ice cores reveal a similar temperature pattern. All the available data indicate we are warmer now than at any other time in the last 1,000 years.
Based on the available data, there is a growing consensus among scientists that global warming is real. We are getting warmer – and this fact needs to be the starting point for any subsequent discussion about global warming.
If you are not yet convinced – or even if you are – look at the data for yourself. Learn how scientists take the Earth’s temperature, how they infer temperature data from tree rings, from coral reefs, from ice cores.
Start with the evidence, and follow that evidence wherever it leads.