Guest post: Global warming edges closer to Paris Agreement 1.5C limit


The latest forecast (pdf) from the World Meteorological Organization (WMO) suggests that over the next five years there is a 24% chance of the global average temperature exceeding 1.5C above pre-industrial levels for at least one year. 

While this is another reminder of the Earth’s rising temperatures, it is important to note that it would not mean that the Paris Agreement ’s long-term goal to limit warming to 1.5C will have been breached. 

This is because the warming limit refers specifically to long-term human-caused warming and not the added effect of natural fluctuations in the climate.

In this article, I delve into the details of the Paris Agreement 1.5C limit, showing why they matter and what the risks are if they are misconstrued.

Human-caused warming

Under the Paris Agreement, almost all the countries of the world have committed to “pursue efforts to limit the global average temperature increase to 1.5C above pre-industrial”. 

As a previous Carbon Brief guest post has noted, the Paris Agreement itself does not spell out explicitly how such temperature limits should be used in climate policy and practice. However, a common interpretation of this is that 1.5C refers here to the long-term global warming attributable to human influence – and not the added effect of natural climate variability.

This is important here because the WMO five-year forecast does encompass natural climate variability on top of the long-term temperature trend. These forecasts are produced with a large set of computer models that calculate weather and ocean processes over the coming few years, including natural fluctuations as well as human impacts. 

Since weather cannot be precisely predicted more than a few days ahead, these forecasts give the probabilities of particular changes. Hence they can be used to estimate the chances of global temperature changes reaching specific levels such as 1.5C. But importantly, they do not refer to the Paris target of limiting the long-term human-caused global warming to 1.5C, as they are predicting natural changes as well as human impacts. They do, however, show how close we are getting to breaching this limit.

The blue band in the chart below shows the WMO forecast for the next five years, with the darker blue shading indicating higher probabilities. The green bands show testing of the forecast method against observations of past global temperatures (black line). 

The forecast suggests that annual global temperatures are likely to be at least 1C warmer than pre-industrial levels in each of the coming five years – and they are all very likely to fall within the range 0.91-1.59C.

The chance of at least one year exceeding 1.5C above pre-industrial levels is 24%, with a very small chance – just 3% – of the five-year average exceeding this threshold.

Observed and predicted changes in annual...

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