CLEARING UP THE CLIMATE DEBATE: CSIRO’s James Risbey explains why it’s not “alarmist” to describe the threat of climate change to the public and how the climate system will respond to half measures.

With many issues to be considered in setting a climate policy one can end up wondering what the role of climate science is in all this.

After all, climate science doesn’t tell us what to do. It doesn’t tell us whether to have a carbon price or where it should be set. Those decisions ultimately involve a range of normative and deliberative issues which are beyond the scope of climatology.

Climatology can tell us, however, what is likely to happen if we don’t act, or if we don’t act with sufficient speed to keep total emissions within specific carbon allocations.

There is no single threshold above which climate change is dangerous and below which it is safe. There is a spectrum of impacts. But some of the largest impacts are effectively irreversible and the thresholds for them are very near.

In particular, the melting and breakdown of polar ice sheets seems to be in the vicinity of a couple of degrees warming. This expectation is based on current high rates of mass loss from the ice sheets compared to relative stability through the Holocene (the past 10,000 years) and on past ice sheet response in periods such as the Pliocene (a few million years ago) when the Earth was a couple of degrees warmer than preindustrial times (and sea level up to 25m higher).

We have already had about 0.8°C warming globally, with another third of a degree locked in by the inertia of the climate system.

That leaves, somewhat optimistically, perhaps a degree or so of wiggle room. Translating that into carbon emissions, if we wish to keep the total warming below about 2°C (with 50% chance), then we have a total global carbon emission allocation of between about 800 and 1000Gt carbon.

We have already emitted about 550Gt, leaving perhaps another 250–450Gt. Current global emissions are about 10Gt per year, growing at roughly 3% per year.

That leaves a few decades at present rates before having committed to 2°C warming and crossing the expected thresholds for ice sheet disintegration. And that is for a 50% chance of not crossing the 2°C threshold. For more comfortable odds of staying within the threshold, the total carbon allocation drops and so the time to threshold is even shorter.

Surely this estimate is vastly uncertain?

Everything has some uncertainty, but the uncertainty in this case lies mostly in the timing, not in the essential result. Ice sheets are sensitive to warming somewhere in this vicinity of temperature change and the climate system will yield 2°C warming somewhere in the vicinity of 800–1000Gt of carbon emissions.

If the climate is a bit less sensitive than we think then we might have a little bit more wiggle room than the 250–450Gt allotment, but not much, and we’ll exceed that allotment very soon thereafter anyway.

We’re only a few decades away from a major tipping point, plus or minus only about a decade. The rate at which the ice sheets would melt is fairly uncertain, but not the result that says we are very close to a tipping point committing to such melt and breakdown.

If we were to keep remaining emissions inside the 250–450Gt carbon allocation, we would need to take account of the inertia in energy systems and infrastructure, which set some limits on the maximum rate that emissions can be reduced.

To stay within the budget, we can’t hope to emit 10Gt a year (the present emissions rate) for the next thirty years and then reduce emissions suddenly to zero. Rather, net emissions would need to be phased down to zero to stay within the budget.

The longer stringent emissions reductions are delayed, the more drastic they must be to stay within the 250–450Gt budget. With more than a small delay, the reductions needed are faster than can be achieved in turning over the stock of emitting infrastructure.

Thus, if we were to stay within this budget, dramatic emission reductions would have to begin now. Delayed action on stringent emissions reductions almost certainly implies overshooting the thresholds and locking in vast long term impacts.

Is it irresponsible or “alarmist” of climatologists to point this out? The science brief for policy is not to prescribe policies, but to point out the implications of pursuing or not pursuing particular courses of action.

Pointing out that we are close to one of the largest tipping points imaginable in the climate system is well within the remit of science. It’s not alarmist to describe the threat accurately; it’s alarming if the political and social culture can’t absorb this.

Sociologists tell us that it is easier to motivate people for climate policies by focusing on the benefits of acting (the carrot) rather than on the costs of not acting (the stick).

As such, they suggest focusing on a positive vision and the good outcomes associated with addressing climate change. While this may be the right strategy, the appeal to benefit comes with no timetable and no particular sense of urgency.

It is the knowledge of climate thresholds and emission rates that sets the timing issues. Science provides the stick, which is the statement of consequence of not reducing carbon dioxide emissions in rapid order. The carrot might be the best way to get us moving, but the stick sets the beat.

Whatever motivations we use to enact climate policy, the climate will respond to our emissions. Emissions policies must therefore be measured by the effects they will have on the climate (among other things).

Policy measures that do not provide the ability for a stringent draw down of carbon emissions on the short time scales implied by the 250–450Gt carbon budget vastly increase the likelihood of crossing critical climate thresholds.

This is the third part of our series Clearing up the Climate Debate. To read the other instalments, follow the links below: