Media coverage today of the launch of the 5th Assessment Report of the IPCC has again said that global warming is “unequivocal” and that the pause in warming over the past 15 years is too short to reflect long-term trends.
Over recent days some commentators have criticised climate models for not predicting the pause. It’s good to see this being addressed, and so begin to clarify the difference between climate model projections and predictions.
We should not confuse climate prediction with climate change projection. Climate prediction is about saying what the state of the climate will be in the next few years, and it depends absolutely on knowing what the state of the climate is today. And that requires a vast number of high quality observations, of the atmosphere and especially of the ocean.
Whilst the last decade has seen a rapid increase in good observations of the surface and upper ocean, thanks to Argo floats, we have very few for the deep ocean. Without these requisite observations to initialise, i.e. set running, a climate prediction, it is impossible to have predicted the current pause, however good the climate models.
On the other hand, climate change projections are concerned with the long view; the impact of the large and powerful influences on our climate, such as greenhouse gases. Projections capture the role of these overwhelming influences on climate and its variability, rather than predict the current state of the variability itself.
The IPCC model simulations are projections and not predictions; in other words the models do not start from the state of the climate system today or even 10 years ago. There is no mileage in a story about models being ‘flawed’ because they did not predict the pause; it’s merely a misunderstanding of the science and the difference between a prediction and a projection.
As the IPCC states in line with our three papers on the pause, the deep ocean is likely a key player in the current pause, effectively ‘hiding’ heat from the surface. Climate model projections simulate such pauses, a few every hundred years lasting a decade or more; and they replicate the influence of the modes of natural climate variability, like the Pacific Decadal Oscillation (PDO) that we think is at the centre of the current pause.
The Daily Telegraph today also covers the science of the pause.
Critically there is ever more confidence that the world is warming as a result of human actions, and limiting climate change would require substantial and sustained reductions in emissions of carbon dioxide (CO2) and other greenhouse gases.