By Kate Abnett

GLASGOW (Reuters).- World leaders stressed at the U.N. summit in Glasgow the importance of limiting global warming to 1.5° Celsius.

The 2015 Paris Agreement commits countries to limit the global average temperature rise to well below 2°C above pre-industrial levels, and to aim for 1.5°C.

Scientists have said crossing the 1.5°C threshold risks unleashing far more severe climate change effects on people, wildlife and ecosystems.

It is necessary to reduce the global CO2 emissions almost by half by 2030, from 2010. This must be done by 2050. Scientists, activists, and financiers at COP26 debate how they can achieve this ambitious goal and what to pay.

But what is the difference between 1.5°C and 2°C of warming? We asked scientists for their explanations:

WHERE ARE YOU NOW?

Already, the world has heated to around 1.1°C above pre-industrial levels. The last 40 years were hotter than any decade before 1850.

Climate scientist Daniela Jacob from the Climate Service Center Germany said that “we have never seen such a rapid global warming within a short time.” Half a degree is more extreme weather. It can happen more frequently, be more intense or last longer.

In China, and Western Europe this year, hundreds died when torrential rainfalls overwhelmed the country. As the Pacific Northwest temperatures reached new records, hundreds more lost their lives. Greenland was the victim of massive melting, Siberia and Mediterranean wildfires, while parts Brazil were hit by a record drought.

Climate scientist Rachel Warren from the University of East Anglia said that climate change already has an impact on every region of the planet.

HEAT, RAIN AND DROUGHT

More warming to 1.5°C and beyond will worsen such impacts.

“For every increment of global warming, changes in extremes become larger,” said climate scientist Sonia Seneviratne at ETH Zurich.

Heatwaves, for example, would be more common and severe.

An extreme heat event that occurred once per decade in a climate without human influence, would happen 4.1 times a decade at 1.5°C of warming, and 5.6 times at 2°C, according to the U.N. climate science panel (IPCC).

Let warming spiral to 4°C, and such an event could occur 9.4 times per decade.

Additionally, warmer temperatures can retain more water. This increases flood dangers. Evaporation is also increased, which can lead to severe droughts.

ICE (NYSE), SEAS and CORAL REEFS

The difference between 1.5°C and 2°C is critical for Earth’s oceans and frozen regions.

“At 1.5°C, there’s a good chance we can prevent most of the Greenland and west Antarctic ice sheet from collapsing,” said climate scientist Michael Mann at Pennsylvania State University.

This would limit the rise in sea level to just a few feet before the end of this century, which is still quite a significant change considering that it would cause erosion and flood some coastal cities and states.

But blow past 2°C and the ice sheets could collapse, Mann said, with sea levels rising up to 10 metres (30 feet)- though how quickly that could happen is uncertain.

Warming of 1.5°C would destroy at least 70% of coral reefs, but at 2°C more than 99% would be lost. It would decimate fish habitats, and the livelihoods of communities who depend on them for their food supply and income.

FOOD, FORESTS and DISEASE

Warming of 2°C, versus 1.5°C, would also increase the impact on food production.

Simon Lewis, University College London climate scientist said that if you experience crop failures in several breadbaskets at once you can see extreme food prices spikes as well as hunger and famine throughout large swathes across the globe.

Warmer temperatures could lead to mosquitoes carrying diseases like malaria or dengue fever expanding across greater areas. But 2°C would also see a bigger share of insects and animals lose most of their habitat range, compared with 1.5°C, and increase the risk of forest fires – another risk to wildlife.

Tipping Points

As the world heats up, the risk increases that the planet will reach “tipping points”, where Earth’s systems cross a threshold that triggers irreversible or cascading impacts. The exact date of these “tipping points” is unknown.

Reduced rainfall and droughts could lead to the Amazon system collapsing (NASDAQ:). This would release CO2 into the atmosphere instead of storing it. The Arctic permafrost might be heated, and long-frozen biomass could begin to decay. This would release large quantities of carbon emissions.

Lewis explained that it was risky to keep emittedting fossil fuels because there is a greater chance of us going over one tipping point.

BEYOND 2°C

So far, the climate pledges that countries have submitted to the United Nations’ registry of pledges put the world on track for 2.7°C of warming. The International Energy Agency said Thursday https://www.reuters.com/business/cop/net-zero-methane-pledges-push-world-near-paris-climate-goal-iea-2021-11-04 that new promises announced at the COP26 summit – if implemented – could hold warming to below 1.8°C, although some experts challenged that calculation. We don’t know if these promises will be realized in the real world.

Warming of 2.7°C would deliver “unliveable heat” for parts of the year across areas of the tropics and subtropics. According to scientists, extreme weather will overwhelm most cities’ ability to handle it, increasing biodiversity and food security will lead to a significant decline in biodiversity.

“If we can keep warming below 3°C we likely remain within our adaptive capacity as a civilization, but at 2.7°C warming we would experience great hardship,” said Mann.