What difference would just half a degree of global warming make?

A new report from the Intergovernmental Panel on Climate Change (IPCC) shows that half a degree of warming matters—a lot.

As part of the historic Paris Agreement on climate change, countries committed to keep global warming well below 2˚C (3.6˚F) above pre-industrial levels while trying to limit temperature increase to 1.5˚C (2.7˚F). Based on a request by governments, the IPCC, a collection of world’s leading climate scientists, took stock of how the impacts of a 1.5˚C temperature limit differ from 2˚C, as well as the differences between emissions pathways for achieving these two temperature goals. Their findings show that the world will face severe climate impacts even with 1.5 degrees of warming, and the effects get significantly worse with 2 degrees. The world has already witnessed about 1˚C of temperature rise and is on track to exhaust the carbon budget associated with 1.5˚C by 2030.

Here’s a breakdown of the differences between a 1.5˚C and a 2˚C world:

Extreme Temperature

Average and extreme temperatures will be higher in all inhabited areas under 2°C of warming versus 1.5°C. For example, under 1.5°C of warming, almost 14 percent of the world’s population would be exposed to severe heat waves at least once in five years. In contrast, under 2°C of warming, 37 percent of the world’s population would be exposed to severe heat waves at least once in five years.

Drought

The probability of drought and risks to water availability may be substantially reduced if warming is limited to 1.5°C. For example, the risk of increased drought magnitude and frequency are significantly larger under 2°C of warming in the Mediterranean and southern Africa than under 1.5°C.

Heavy Precipitation and Flooding

High latitude and mountainous regions, as well as Eastern Asia and Eastern North America, are projected to experience heavier precipitation under 2°C of warming than under 1.5°C. While 1.5°C can lead to increased runoff and floods in some regions compared to today’s conditions, 2°C could lead to even more.

Ice-Free Arctic

With 1.5°C, the report finds it is very likely to have one sea-ice-free summer every 100 years; at 2°C, the frequency increases to at least one every 10 years. This can lead to more heat being absorbed, impacts to ocean circulation, and have consequence for winter weather in the Northern hemisphere.

Sea Level Rise

With 1.5°C of warming, sea level rise would be 0.4 meters (1.3 feet) in 2100, compared to levels in 1986-2005. At 2°C, it would be 0.46 m (1.5 ft) in 2100.

The risk of flooding is also greater with a higher temperature rise. With 1.5°C of warming by 2100, up to 69 million people could be exposed to flooding (assuming no adaptation and current population). Up to 79 million could be exposed under 2°C of warming.

Also, if the rate of sea level rise is slower, there can be more opportunities for communities to adapt.

Species Loss

At 2ºC warming, 18 percent of insects globally, 16 percent of plants and 8 percent of vertebrates are projected to lose more than half of their ranges. With 1.5°C of warming, this is reduced by two-thirds for insects, and by half for plants and vertebrates.

Other factors that lead to losses of species, such as forest fires and the spread of pests and diseases, also decrease if warming stays at 1.5°C.

Terrestrial Ecosystems

Ecosystems are expected to transform with greater warming. For example, under 2°C of warming, 13 percent of the Earth’s land area is projected to witness biome shifts (such as changing from tundra to forest), or transformation. With 1.5°C of warming, this risk is lowered to 4 percent of Earth’s land area.

Under higher temperatures, permafrost is at a much greater risk for melting, which would lead to the release of its stored carbon into the atmosphere.

With warming of 2°C, 35-47 percent of the Arctic’s permafrost would thaw by 2100, an area of land that is three-quarters the size of Australia. If warming is limited to 1.5°C, the extent of thawing permafrost drops to 4.8 million km2. or about 21-37 percent of the total permafrost area..

Oceans

Ocean ecosystems are already transforming and will change dramatically with just 1.5°C of warming. However, limiting warming to 1.5°C can stave off many of the impacts that higher temperatures would bring.

For example, coral reefs are projected to decline by 70-90 percent with warming greater than 1.5°C. With an additional half degree of warming, more than 99 percent losses are expected. Loss of fishery productivity at low latitudes, acidification, dead zones and other dangerous conditions are projected to be more pronounced with warming higher than 1.5°C. For example, one study cited in the report found that the global annual catch from marine fisheries declined by 1.5 million tonnes under 1.5°C of warming; under 2°C. that loss grew to 3 million tonnes.

Food Security

The risks of food shortages are projected to be lower in the Sahel, southern African, Mediterranean and Amazon regions under 1.5 °C of warming than they would be at 2°C of warming. Fisheries and aquaculture also experience lower risks if warming stays at 1.5°C.

Health

Risks to human health, including heat-related morbidity and mortality in urban areas, are lower with 1.5°C of warming than 2°C.

Economic Growth

Economic losses are greater as temperatures rise, with middle income countries (Africa, southeast Asia, India, Brazil, Mexico) projected to be affected the most. For example, if warming is limited to 1.5°C, global GDP losses will be 0.3 percent by 2100. With 2°C of warming, losses would be 0.5 percent.

How much harder is it to limit warming to 1.5˚C vs. 2˚C ?

Meeting both the 1.5°C and 2°C limit will require unprecedented transformation across all economies, industries and geographies.

For limiting warming to 1.5°C with no or limited overshoot, the world will need to reduce its annual emissions to 25-30 GtCO2e on average in 2030. The world is currently on track to emit more than double this amount by 2030 (52-58 GtCO2e). Limiting warming to 2°C would require reducing annual emissions by about 20 percent below 2010 levels in 2030; for 1.5°C, emissions will need to drop by 40-50 percent. Carbon dioxide emissions will need to reach net-zero around 2050 to limit warming to 1.5°C and around 2075 for 2°C.

Setting Sights on 1.5˚C

Achieving 2˚C will avoid many catastrophic impacts from climate change, but the consequences will be significantly worse than if we can limit global warming to 1.5˚C. Adaptation needs also grow as temperature rises. Saving this half a degree will require tremendous effort. But as the report today shows, the effort will be well worth the reward of more secure communities, ecosystems and economies.