The recent slowdown in the rise of global average air temperatures during the first decade of the 21st century is being used as a touchstone argument for those who deny the science of global warming. But in research published today in Nature Climate Change, I and others show that the “hiatus” is just a blip on the radar compared to the long-term warming we have in store.
The slowdown or “hiatus” in warming refers to the period since 2001, when despite ongoing increases in atmospheric greenhouse gases, Earth’s global average surface air temperature has remained more or less steady, warming by only around 0.1C. This contrasts with the 1990s, when warming reached more than 0.2C.
But by looking at around 200 projections from climate models, and separating those that capture the slowdown from those that do not, we have shown that the slowdown does not affect long-term warming projections in any measurable way.
More extreme climate change contrarians have seized upon this so-called “pause” as a sign that global warming has stopped altogether, or that we are headed into a period of global cooling. Considering that global average temperatures are still rising by a variety of measures, just more slowly, and as the last time we had a coldest year on record was 1909, this is quite a leap of faith.
Unfortunately using global average surface air temperatures as a measure of total warming ignores the fact that most of the heat (more than 93%) goes into our oceans, which continue to warm without any sign of a pause, as you can see below.
Brakes on global warming?
Global average surface air temperatures only reflect the heat present in the atmospheric layer immediately above the land/ocean surface. As heat gets sloshed around the oceans by processes such as El Niño, the overlying atmosphere responds by tracking these heat variations.
As a result, surface temperature is strongly affected by natural variability. Beyond year-to-year variability such as El Niño there are decade-to-decade changes, such as the Interdecadal Pacific Oscillation, which has been shown to have a marked impact on global temperature rise.
In particular the negative phase of the Interdecadal Pacific Oscillation can lead to dramatically increased trade winds and fewer El Niños – as has been occurring since 2001. The modulation of these processes can significantly impact global average temperatures.
It is no coincidence that the only year outside the 21st century that remains in the top 10 hottest years on record is 1998. That year saw the strongest El Niño ever recorded – a natural variation that added considerable heat to the 1998 result.
Ever since then climate change deniers have loved to produce graphs that start in 1998, although when 2005 broke through as the new warmest year on record, another new favourite start year was born. Cherry-picking data at its worst.
And just for the record: 2010 then effectively tied with 2005 for the hottest year, and now 2014 has beaten the lot. And all the while the coldest year, 1909, languishes as a fading memory of a time long past, predating human’s undeniable footprint on climate. It’s a record that is certain to stand for a long time to come.
While 1998 was a warm year for the surface atmosphere, in the oceans, where most of the warming is going, the story is different. Looking at ocean temperatures down to 2 km, 1998 does not even make the cut for the top 10. Instead, total annual average ocean heat content has increased steadily during the hiatus, at quite a confronting rate given that this metric is closely tied to global sea-level rise.
The strong influence of natural variability on surface air temperatures is the reason that climate researchers regularly point out that any record shorter than around 20-30 years is not useful for detecting long-term trends associated with anthropogenic warming.
Step back from these short time scales and look at the past century to see the clear human influence.
Sifting the signal from the noise
Climate modellers have run hundreds of future warming simulations. Some of these simulations capture the slowdown if they happen to be in the right phase of the Interdecadal Pacific Oscillation.
But if you average all the model projections, the warming predicted for the past decade outpaces observations. This has been argued as evidence that scientists have overstated the threats posed by increasing concentrations of greenhouse gases.
Yet the climate science community has repeatedly shown compelling evidence that this slowdown reflects decade-to-decade variability.
Until now, however, no evaluation has been made of the possible consequences for long-term projections. Specifically, if the variability controlling the current hiatus is linked to longer-term sequestration of heat into the deep ocean, this might require us to recalibrate future projections.
With this in mind, we decided to test whether 21st century warming projections are altered in any way when considering only simulations that capture a slowdown in global surface warming, as observed since 2001.
As you can see in our paper we looked at this by separating all available future projections into two groups - those that captured the current slowdown and those that did not. We then compared the warming throughout the 21st Century for both groups.
We considered two well-known emissions scenarios taken from the latest IPCC report. The first scenario assumes greenhouse gas concentrations continue to rise unabated through the 21st century. The second assumes emissions are reduced to address global warming, peaking by 2040 before declining sharply.
It turns out that the difference between the “slowdown” set and the rest of the projections was negligible by the mid-21st century. And the longer the projections go on, the smaller the difference gets.
Under the high emissions scenario, for example, the difference in average projected end-of-century warming between the two groups of models is less than 0.1C; a tiny fraction of the projected 5C global warming if emissions are not curbed.
This clearly shows that the impact of the current hiatus is effectively non-existent in the context of long term warming.
Don’t wait for the warming to return
It is not news to climate scientists that natural variability impacts the short-term movement of global average temperatures in both directions. We have seen short-term pauses and even short-term reversals in global temperature rise in the past century, against the backdrop of an unambiguous long term warming trend.
We have also seen periods of more rapid surface atmosphere warming. In effect the surface warming comes in spurts, facilitated by phase shifts in the Interdecadal Pacific Oscillation.
And over time, despite the cycles of warming and cooling due to this oscillation, despite solar minima, despite cooling from volcanic eruptions and cooling from the massive loading in the atmosphere of anthropogenic aerosols, the trend in temperatures above the noise clearly continues upwards.
Our research shows that limiting the projections to models that capture the current slowdown in no way alters long-term projections. We can thus place extra confidence in the synthesised projections of the IPCC.
If we continue on our current high emissions trajectory the world will have no chance of staying under the 2C threshold that the federal government has committed to.
Limiting warming to 2C is a laudable aspiration – one that gives us a decent chance (not certainty) of avoiding dangerous interference with the climate system. Unfortunately if we hang around talking about a 15-year slowdown for too long, the chance to limit warming to less than the 2C threshold will rapidly disappear.