The difference half a degree of global warming can make

The ‘Global Warming of 1.5˚C’ report of the Intergovernmental Panel on Climate Change highlights several climate change impacts that could be avoided by limiting global warming to 1,5˚C, compared with 2˚C or more. For example, at 1,5˚C warming, the Arctic Ocean is likely to be free of sea ice in summer just once a century on average, whereas at 2°C global warming, the likelihood rises to once a decade.

The difference half a degree of global warming can make

Human activities are estimated to have caused 1°C of global warming above pre-industrial levels, with a likely range of 0,8°C to 1,2°C.

Current models show that global warming is likely to reach 1,5°C between 2030 and 2052 if it continues to increase at the current rate.

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In addition, warming from emissions originating in human activity from the pre-industrial period to the present will persist for millennia and continue to cause long-term changes in the climate system, such as sea-level rise. Impacts on natural and human systems from global warming have already been observed.

Many land and ocean ecosystems have changed due to global warming. Future climate-related risks depend on the rate, peak and duration of warming.

In the aggregate, they will be larger if global warming exceeds 1,5°C before returning to that level by 2100 than if global warming gradually stabilises at 1,5°C, especially if the peak temperature is high, for example about 2°C.

Some impacts, such as the loss of an ecosystem, may be long-lasting or irreversible.

Potential impacts and risks
Some climate characteristics will be influenced by the conditions in particular regions and by the extent to which temperatures rise due to global warming.

They include: mean temperature in most land and ocean regions; hot extremes in most inhabited regions; heavy precipitation in several regions; and the probability of drought and precipitation deficits in some regions.

It is projected that, by 2100, global mean sea-level rise will be about 0,1m lower with global warming of 1,5°C than with a figure of 2°C. The sea level will, however, continue to rise well beyond 2100, and the magnitude and rate of this rise will depend on future emission pathways.

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A slower rate of sea-level rise will enable greater opportunities for adaptation in the human and ecological systems of small islands, low-lying coastal areas and deltas.

In addition, a reduction of 0,1m in global sea-level rise implies that up to 10 million fewer people would be exposed to related risks, based on population in the year 2010 and assuming no adaptation.

On land, impacts on biodiversity and ecosystems, including species loss and extinction, are also projected to be lower at 1,5°C of global warming than at 2°C.

Of 105 000 species studied, 6% of insects, 8% of plants and 4% of vertebrates would lose over half of their climatically determined geographic range if there was global warming of 1,5°C, compared with 18% of insects, 16% of plants and 8% of vertebrates in the case of 2°C global warming.

Under the more favourable scenario, the impacts on terrestrial, freshwater and coastal ecosystems will also be reduced, and these systems will retain more of their services to humans.

Limiting global warming to 1,5°C rather than 2˚C is projected to reduce increases in ocean temperature as well as associated increases in ocean acidity and decreases in ocean oxygen levels.

Consequently, this will reduce the risk to marine biodiversity, fisheries and ecosystems, and their functions and services to humans.

There is high confidence that the probability of a sea ice-free Arctic Ocean during summer would be substantially lower at global warming of 1,5°C than at 2°C.

With 1,5°C of global warming, an average one sea ice-free Arctic summer is projected per century.

This likelihood is increased to at least one per decade with 2°C global warming. To illustrate the risks of climate-induced impacts on marine diversity, coral reefs, for example, are projected to decline by a further 70% to 90% at 1,5°C and by up to 99% at 2˚C.

Daytime high temperatures in most mid-latitude inhabited regions are projected to increase by up to 3°C at global warming of 1,5°C and about 4°C at 2°C, and night-time extremes in high latitudes will warm by up to 4,5°C at 1,5°C and about 6°C at 2°C.

The number of hot days is projected to increase in most regions, especially the tropics. Risks from droughts and precipitation deficits, heavy precipitation events and precipitation associated with tropical cyclones and flood hazards are projected to be higher at 2°C global warming than at 1,5°C in certain regions.

Food security and health
Climate-related risks to health, livelihoods, food security, water supply, human security and economic growth will increase with global warming of 1,5°C and increase further with 2°C.

Populations at disproportionately higher risk of adverse consequences include disadvantaged and vulnerable populations, some indigenous peoples, and local communities dependent on agricultural or coastal livelihoods.

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Regions at disproportionately higher risk include Arctic ecosystems, dryland regions, small island developing states, and least developed countries.

Poverty is expected to increase in some populations as global warming increases, but limiting the figure to 1,5°C could reduce the number of people both exposed to climate-related risks and susceptible to poverty by several hundred million by 2050.

Nevertheless, any increase in global warming is projected to affect human health, with negative consequences such as heat-related morbidity and mortality, and ozone-related mortality.

Risks from some vector-borne diseases, such as malaria and dengue fever, are expected to increase with warming from 1,5°C to 2°C; the danger includes potential shifts in the geographic range of these diseases.

Food security is also affected by global warming. But limiting warming to 1,5°C would result in smaller net reductions in yields of maize, rice, wheat and potentially other grain crops, particularly in sub-Saharan Africa, Southeast Asia, and Central and South America.

Livestock are also projected to be adversely affected due to changes in feed quality, the spread of diseases, and water resource availability.

Depending on future socio-economic conditions, limiting global warming to 1,5°C may reduce the proportion of the world population exposed to a climate change-induced increase in water stress by up to 50% compared with 2°C global warming. There is considerable variability between regions, however.

The way forward
Every additional fraction of warming matters, particularly as warming of 1,5˚C or higher increases the risk of long-lasting or irreversible changes, such as the loss of some ecosystems.

Therefore limiting global warming to 1,5˚C is crucial. Limiting global warming will also give people and ecosystems more room to adapt and remain below the risk thresholds.

The report also examines pathways available to limit warming to 1,5˚C, what it would take to achieve them, and what the consequences could be.

Limiting global warming to 1,5°C will require rapid and far-reaching transitions in land, energy, industry, buildings, transport and cities.

Global net human-caused emissions of carbon dioxide (CO2) will need to fall by about 45% from 2010 levels to 2030, and reach ‘net zero’ in 2050.

Allowing the global temperature to temporarily exceed 1,5˚C will mean a greater reliance on techniques that remove CO2 from the air to return global temperature to below 1,5˚C by 2100.

The effectiveness of such techniques is unproven at a large scale and some may carry significant risk for sustainable development. The good news is that some of the actions needed to limit global warming to 1,5˚C are already underway around the world, but they will need to accelerate.

The views expressed in our weekly opinion piece do not necessarily reflect those of Farmer’s Weekly.

This is an edited excerpt from the ‘Global Warming of 1.5˚C’ report and its supporting documents.