World may overshoot 1.5°C warming target: What it will take to bring global temperatures back down

Instead, the world could enter an “overshoot” period – when global temperatures rise above 1.5°C before eventually falling back below it. UNEP says that remains possible, but highly uncertain, and would require rapid cuts in greenhouse gas emissions alongside large-scale removal of carbon dioxide from the atmosphere.

“1.5 target is still the goal. But now we need to approach it differently from above,” UNEP Executive Director Inger Andersen said.

The report titled “Limiting Overshoot: How to Navigate Exceedance” says governments now need to focus on two things: limiting how high temperatures rise and reducing the amount of time the world remains above 1.5°C.

Why does crossing 1.5°C matter?

The 1.5°C threshold comes from the 2015 Paris Agreement, under which governments agreed to pursue efforts to limit the increase in global average temperature to 1.5°C above pre-industrial levels.

The threshold is based on a longer-term global average, rather than a single year’s temperature. Global temperatures are already about 1.4°C above pre-industrial levels, reported AP.

UNEP’s latest assessment says the world is likely to cross 1.5°C within the next few years.

That does not mean there will be a sudden climate catastrophe on the day the threshold is crossed. University of Victoria climate scientist Andrew Weaver, who was not involved in the UNEP report, described 1.5°C as not being “some magical climate cliff” where everything changes at once, according to AP report.

But higher temperatures increase climate risks. These include more extreme weather, ecosystem damage, ice loss and growing pressure on food and water supplies.

UNEP says every year of delay in cutting emissions makes the eventual problem harder to manage. Climate scientist Joeri Rogelj said in the report that every five years of delay in reducing emissions adds at least 0.1°C to peak warming.

That means the height of the eventual temperature peak matters even if the world eventually manages to bring temperatures back below 1.5°C.

How could temperatures come down?

UNEP’s strategy involves two actions that have to happen together: rapidly reducing greenhouse gas emissions and removing carbon dioxide from the atmosphere. Cutting emissions limits additional warming. Carbon removal addresses some of the carbon dioxide that has already accumulated in the atmosphere.

The report says carbon dioxide removal will become “critical” in an overshoot scenario. But it also makes clear that carbon removal cannot substitute for deep emissions cuts.

Forests are one established form of carbon removal because trees absorb and store carbon dioxide as they grow. But UNEP estimates that tree planting could reduce global temperatures by only about 0.1°C at most.

That means forests alone cannot bring the world back below 1.5°C if warming significantly overshoots the target.

Other approaches include technologies designed to remove carbon dioxide directly from the air and store it. But these methods face major questions over their cost, scale and reliability.

The higher the world allows temperatures to rise, the greater the amount of carbon removal that may eventually be required.

UNEP’s most optimistic scenario puts peak warming at around 1.8°C above pre-industrial levels, assuming countries implement their existing national climate pledges. Under current policies, the report projects median warming of about 2.6°C by 2100.

What happens if the world overshoots 1.5°C?

Returning below 1.5°C would not erase the damage caused during the period of overshoot. The UNEP report expects decades of higher weather-related risks, rising seas, melting ice and ecosystem damage while temperatures remain above the threshold.

It also warns about potential climate tipping points, including major changes involving the Greenland and West Antarctic ice sheets, the Amazon rainforest and the Atlantic Ocean circulation system. Some of these changes could have effects that do not simply reverse when global temperatures fall.

UNEP also warned that some locations could become “more or less uninsurable or even uninhabitable”. 

The effects would extend beyond the environment. Higher temperatures can put additional pressure on food production and water availability, while extreme weather can increase costs for governments, households and businesses.

That is why bringing temperatures back down is not equivalent to avoiding 1.5°C in the first place. Some losses may already have occurred by the time temperatures decline.

Can the world still avoid the worst outcomes?

UNEP’s message is not that crossing 1.5°C makes climate action pointless. It is that every fraction of a degree still matters.

Richard Betts, a climate scientist at the University of Exeter, described the situation as a “reality check”, saying there was still much that could be done to limit warming.

The same applies to adaptation. Governments will need to prepare for climate impacts that can no longer be avoided, particularly in poorer populations that often have fewer resources to respond to extreme heat, floods and droughts.

But the longer-term challenge is bringing temperatures back down after the peak. That would require sustained emissions reductions and carbon removal over decades.

The world may no longer be able to prevent a temporary breach of the 1.5°C threshold. But it can still influence how high temperatures rise, how long they stay there and how much additional damage occurs.

The earlier emissions fall, the lower the potential peak. And the lower the peak, the smaller the scale of carbon removal and climate damage that may have to be dealt with later.