The Intergovernmental Panel on Climate Change has warned the effect of unchecked global warming could be “severe, pervasive and irreversible”.

To get an idea of how global temperatures could evolve in the next 85 years, the IPCC looks at a range of scenarios, or pathways. In all but the IPCC’s most stringent mitigation pathway, the world looks on course for dangerous levels of warming.

So how did climate skeptic campaigner, Matt Ridley, come to the opposite conclusion in an article last week? He argues that even the IPCC’s most severe scenario doesn’t suggest dangerous levels of warming.  We take a look at the IPCC’s pathways to understand how it projects future warming – and where Ridley went wrong.

Warming scenarios

The IPCC looks at a wide range of potential futures based on how big or small countries’ collective greenhouse gas emissions are. To examine these, it developed a range of emissions pathways – each known as Representative Concentration Pathways (RCP).

Ridley describes the IPCC’s four pathways in his recent article, and claims none of them imply dangerous levels of warming. He begins:

“Three of the models show moderate, slow and mild warming, the hottest of which leaves the planet just two degrees Centigrade warmer than today in 2081-2100. The coolest comes out just 0.8 degrees warmer.”

Ridley’s talking about the IPCC’s three lowest scenarios – leaving out the one where emissions continue unchecked for now. But he seems to have got the numbers wrong.

Changes in average global surface temperature by 2081-2100 for each of the IPCC’s scenarios, relative to 1986-2005. The observed warming to the reference point from pre-industrial times (1850-1900) is an additional 0.61 degrees Celsius. Source: IPCC 5th Assessment Report ( AR5)

In fact, the IPCC’s “coolest” scenario – RCP2.6 – sees temperatures rise on average 1.0 degree above current levels by the end of the century. But that’s only assuming immediate “substantial and sustained” emissions cuts, the IPCC says.

The scenario Ridley labels the “hottest” of the three – RCP6.0 – sees temperatures stabilise at 2.2 degrees warmer than they are now, not two degrees as Ridley suggests. Even minor temperature changes can have a big impact, so an extra 0.2 degrees could be significant.

Two degrees of warming above pre-industrial levels is the point generally accepted by scientists and governments to represent “dangerous” climate change, as Ridley acknowledges. What he neglects to mention is that we’ve already seen 0.61 degrees warming since the start of the industrial revolution.

For RCP6.0, that means a total warming of 2.8 degrees by 2100 – considerably higher than the two degrees Ridley suggests is “the threshold at which warming starts to turn dangerous”.

What’s more, the IPCC suggests we could encounter serious consequences at less than two degrees warming:

“Some unique and threatened systems, including ecosystems and cultures, are already at risk from climate change â?¦ The number of such systems at risk of severe consequences is higher with additional warming of around 1°C …Climate change-related risks from extreme events, such as heat waves, extreme precipitation, and coastal ï¬?ooding, are already moderate … and high with 1°C additional warming”.

‘Implausible’ assumptions

Each RCP makes different assumptions about different aspects of society, growth and how the climate system behaves – including how sensitive the atmosphere is to carbon dioxide.

Having dismissed three of the IPCC’s scenarios, Ridley argues that the IPCC’s fourth and highest warming scenario – RCP8.5 – “cannot produce dangerous warming” either. The scenario makes “very, very implausible” assumptions, he claims.

But Ridley’s assessment is based on some questionable propositions.

First, he suggests the IPCC’s top-end scenario incorrectly identifies how much warming carbon dioxide causes. He argues the amount of warming per doubling of carbon dioxide – something scientists call climate sensitivity – is lower if it’s calculated from observed warming this century, compared to climate model estimates based on how scientists think elements of the climate system interact over longer timescales.

That’s true – but the IPCC assesses all lines of evidence to arrive at  a “likely” range for climate sensitivity. In contrast, Ridley places more weight on methods that point toward lower sensitivity while apparently ignoring all the evidence pointing to higher sensitivity.

By Ridley’s reckoning, RCP8.5 with a lower climate sensitivity would mean an additional 2.1 degrees of warming by 2100, rather than the 3.7 degrees the IPCC estimates. But that increases to 2.7 degrees once warming so far has been take into account – still considerably above the level he acknowledges could be dangerous.

It’s also worth pointing out the IPCC’s scenarios are not forecasts. The pathways illustrate a range of possibilities depending on how society’s energy use evolves. The IPCC makes no judgement on which one is most likely. So arguing a single scenario’s assumptions are implausible – as Ridley does – somewhat misses the point.

Despite his arguments, the IPCC’s models certainly don’t support Ridley’s claim that climate change is “not likely to do much harm”.

Counting the cost

Ridley offers one more argument for not taking action to tackle climate change: the cost of doing so is higher than the cost of the damage climate change may cause. But – as we’ve discussed in detail before – this presents a false choice.

Ridley says:

“The IPCC produced two reports last year. One said that the cost of climate change is likely to be less than two per cent of GDP by the end of this century. The other said that the cost of decarbonizing the world economy with renewable energy is likely to be four per cent of GDP. Why do something that you know will do more harm than good?”

But presenting the figures in this way misrepresents how climate change is likely to affect the economy.

The two per cent figure Ridley refers to does appear in the IPCC’s report, but it doesn’t include all the costs the world might incur as a result of two degrees of warming.

Moreover, it refers to the cost we’re already locked in to paying given the world’s greenhouse gas emissions to date. If the world warms by more than two degrees – which will happen without stringent mitigation  – the costs are likely to be much higher.

The four per cent figure, on other hand, refers to how much the world may have to invest by 2030 to prevent further warming. In order for it to remain “likely” that the two degrees target can be achieved, the IPCC says governments will have to implement policies that will cost between 0.04 and 0.14 per cent of global consumption growth each year. The average cost could be around 0.06 per cent a year, it says.

But that cost is compared to a fictional world where there’s no climate change.

So it’s not a case of the world expending either/or two or four per cent to deal with climate change, as Ridley suggests. The choice we have is whether to invest a fraction of global consumption to curb global warming, or pay the potentially astronomical costs of major climate change.

When the costs of doing nothing – as Ridley recommends – are taken into account, policymakers may decide a investing a fraction of annual global consumption in climate policy is a small price to pay.