How much has the Earth’s temperature changed in the last 100 years? And what does that mean for us?
The surface of the earth has warmed by about 1 degree over the last 100 years. We know this because humans have been making measurements of land and ocean temperatures for several hundred years. By carefully combining these measurements and using mathematical methods to fill in gaps, scientists have been able to track how earth’s temperature has changed since about 1850. To be more precise and consistent, climate scientists have agreed to standardise how they talk about warming. So rather than look at 100 years ago, they use the average temperature between 1850 – 1900 as the background starting point. At the other end, the latest reporting compares this starting point to the average temperature from 2011 – 2020. Between those two time periods (which is slightly more than 100 years), our best estimate is that the earth’s surface has warmed by 1.09 degrees. The earth’s surface is made up of land and sea, and if we break it down, we see that the land surface has warmed more than the sea surface- the land has warmed by 1.59 degrees, while the ocean surface has warmed by 0.88 degrees.
We know the most about how Earth’s surface temperature has changed, because that’s where we have the most measurements from. However, other parts of the earth are also warming. In the atmosphere, temperatures have increased throughout the lower part of the atmosphere, known as the troposphere (roughly from the surface to 15 km above). The ocean has warmed down to at least 2km below the surface, although the strongest warming is at the ocean surface.
What this all means for us is a great question. First, global surface temperatures might not mean that much to us, as we only live in one place. The data for Tasmania, compiled by the Bureau of Meteorology (you can have a look at graphs here) show that Tasmania has followed the global trend pretty closely, and has warmed by about 1 degree over the last 100 years. 1 degree of warming doesn’t sound like very much- after all, a high of 18 is not that different from a high of 17. Certainly in Tasmania this temperature rise hasn’t pushed us to the edge of what the human body can endure. But a rise in temperature is associated with other changes, to things like bushfires, droughts, floods and the movements of plants, animals and even diseases. These have all changed over the past 100 years and we’re still studying how much, how this has affected us and to what extent the changes can be attributed to climate change.
Another thing this means for us is how our ability to explain the temperature change that has occurred gives us confidence that as scientists we have quite a good understanding of how climate works. The graph below shows that the observed warming (black line) is reproduced quite well by climate models, but only if the models include human (red and blue) and natural (green) causes- the natural causes alone can’t reproduce the temperature rise that’s occurred over the last 100 years. This means we can trust the predictions made by the climate models, and this trust should give us confidence to take action to prevent the worst outcomes.
