Thursday 26 January 2017 11.00 GMT
We don’t want the Earth to warm more than 1.5–2°C (2.7-3.6°F) compared to the pre-industrial climate. These targets are not magical; they are expert judgements about what it takes to avoid some of the moreserious effects of climate change. We know the seas will rise (they already are). We know droughts and flooding will get more severe (they already are). We know there will be more heat waves, more intense storms, and ocean acidification (all happening now). We cannot stop some of the changes. But if we keep climate change to these limits, we think we can avoid the worst effects.
Where did these targets come from? Well, I mentioned that they are expert judgements but they are based on science. For instance, we can look into the deep past using ice cores, sediment records, and other tools to see how the past climate changed. We can also look into the future with computer models to predict how the future climate will evolve. Through these tools we can get a sense of how large the impact is if temperatures rise.
The obvious question is, where are we at? How much have temperatures risen since the pre-industrial time period? It might seem like that is a simple question. In fact, groups like NASA in the USA regularly provide temperature data as below. According to this image, the 2016 temperature increase has just hit 1°C (1.8°F). So, it would appear that we have some ways to go before hitting our target, right?
Not so fast. Whenever you see an image like the one below, you should ask what years are the baseline. These graphs are termed “temperature anomaly plots.” They don’t show the actual temperature; rather they show the temperature difference between two time periods. It turns out the figure below is oriented so that it is relative to the time period 1951-1980. So when we say that 2016 had a temperature anomaly of 1°C, we really mean that it was 1°C warmer than the 1951-1980 time period.