How do the impacts of climate change compare between 1.5C and 2C?


Piers Forster: “1.5C is a brave new world. Since the inclusion of the 1.5C limit in the Paris Agreement, there has been something of a flurry of research into the impacts of 1.5C of warming on the planet… Climate scientists were caught napping. “

In fact, as Prof Piers Forster – professor of physical climate change at the University of Leeds and a lead author on chapter two of the special report – wrote in a Carbon Brief guest post at the end of the Paris talks, “climate scientists were caught napping” by the 1.5C limit:

“Before Paris, we all thought 2C was a near-impossible target and spent our energies researching future worlds where temperatures soared. In fact, there is still much to discover about the specific advantages of limiting warming to 1.5C.”

In a recent interactive article, Carbon Brief presented the findings of around 70 peer-reviewed studies showing how the potential impacts of climate change compare at 1.5C, 2C and beyond. The data covers a range of impacts – such as sea level rise, crop yields, biodiversity, drought, economy and health – for the world as a whole, as well as specific regions.

In the special report on 1.5C, chapter one (pdf) notes that climate impacts are already being observed on land and ocean ecosystems, and the services they provide:

“Temperature rise to date has already resulted in profound alterations to human and natural systems, bringing increases in some types of extreme weather, droughts, floods, sea level rise and biodiversity loss, and causing unprecedented risks to vulnerable persons and populations.”

The people that have been most affected live in low- and middle-income countries, the report says, some of whom have already seen a “decline in food security, linked in turn to rising migration and poverty”. Small islands, megacities, coastal regions and high mountain ranges are also among the most affected, the report adds.

In general – and, perhaps, unsurprisingly – the potential impacts of global warming “for natural and human systems are higher for global warming of 1.5C than at present, but lower than at 2C”, the SPM says. The risk are also greater if global temperatures overshoot 1.5C and come back down rather than if warming “gradually stabilises at 1.5C”.

There are a lot of impacts to consider, which is reflected in the fact that chapter three (pdf) on impacts is the longest of the whole report at 246 pages.

In many cases, the IPCC has “high confidence” that there is a “robust difference” between impacts at 1.5C and 2C – such as average temperature, frequency of hot extremes, heavy rainfall in some regions and the probability of drought in some areas.

As an illustration, the report includes a “reasons for concern” graphic that shows how the risks of severe impacts varies with warming levels. The example below shows a section of this graphic showing some of these impacts. The coloured shading indicates the risk level, from “undetectable” (white) up to “very high” (purple).

The graphic shows how warm water corals and the Arctic are particularly at risk from rising temperatures, moving into the “very high” category with 1.5C and 2C of warming, respectively.

How the level of global warming affects impacts and/or risks associated for selected natural, managed and human systems. Adapted from IPCC (pdf)

How the level of global warming affects impacts and/or risks associated for selected natural, managed and human systems. Adapted from IPCC (pdf)

Tropical coral reefs actually get their own specific section in Box 3.4 in chapter three, which emphasises that at 2C of warming, coral reefs “mostly disappear”. However, even achieving 1.5C “will result in the further loss of 90% of reef-building corals compared to today”, the report warns. And short periods (i.e. decades) where global temperatures overshoot 1.5C before falling again “will be very challenging to coral reefs”.

For the Arctic, the report expects that “there will be at least one sea-ice free Arctic summer out of 10 years for warming at 2C, with the frequency decreasing to one sea-ice-free Arctic summer every 100 years at 1.5C”. Interestingly, the report also notes that overshooting 1.5C and coming back down again would “have no long-term consequences for Arctic sea-ice coverage”.

Warming of 1.5C will also see weather extremes become more prevalent across the world, the report says. Increases in hot extremes are projected to be largest in central and eastern North America, central and southern Europe, the Mediterranean region, western and central Asia, and southern Africa. Holding warming to 1.5C rather than 2C will see around 420 million fewer people being frequently exposed to extreme heatwaves, the report notes.

High and low extremes in rainfall are also expected to become more frequent, the report says. The largest increases in heavy rainfall events are expected in high-latitude regions, such as Alaska, Canada, Greenland, Iceland, northern Europe and northern Asia. Whereas in the Mediterranean region and southern Africa, for example, “increases in drought frequency and magnitude are substantially larger at 2C than at 1.5C”.

For global sea levels, increases are projected to be around 0.1m less at 1.5C than at 2C come the end of the century, the report notes, which would mean that “up to 10.4 million fewer people are exposed to the impacts of sea level globally”. However, sea levels will continue to rise beyond 2100, the report says, and there is a risk that instabilities in the Greenland and Antarctic ice sheets triggered by 1.5–2C of warming cause “multi-metre” increases in sea levels in the centuries and millennia to come.

Sea level rise is particularly pertinent for the risks facing small island states, which are covered in Box 3.5. The combination of rising seas, larger waves and increasing aridity “might leave several atoll islands uninhabitable” under 1.5C, the report warns.

Another topic given its own specific box is food security (“Cross-Chapter Box 6”), which is affected in various different ways by climate change, the report says:

“Overall, food security is expected to be reduced at 2C warming compared to 1.5C warming, due to projected impacts of climate change and extreme weather on crop nutrient content and yields, livestock, fisheries and aquaculture, and land use (cover type and management).”

Climate change can exacerbate malnutrition by reducing nutrient availability and quality of food products, the report notes. However, in general, “vulnerability to decreases in water and food availability is reduced at 1.5C versus 2C, whilst at 2C these are expected to be exacerbated, especially in regions such as the African Sahel, the Mediterranean, central Europe, the Amazon, and western and southern Africa”.

— Carbon Brief (@CarbonBrief) October 4, 2018


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