Why a Half-Degree Rise in Temperature Matters to the Planet

The IPCC has warned of extremely serious dangers to our climate if global warming exceeds 1.5°C and reaches 2°C. But many people don't understand how a half-degree can make such a difference. So we explain why such a small increase in Earth’s temperature is such a big deal for the environment, for animals and other living organisms on land and sea, including human beings.
Tidewater Glacier Antarctic Ocean
Melting glaciers could cause sea levels to rise 1-2 meters by 2100. Photo: Jason Auch/CC BY 2.0

Question: Why is everyone talking about limiting global warming to 1.5 degrees Celsius instead of allowing it to reach 2 degrees? I mean, what’s the big deal? How can a half-degree rise in temperature make such a difference? How does it become an ‘extinction-level event’?

Answer: It’s true, a half-degree change is barely perceptible to a human body. But it’s not a human body we’re dealing with. It’s the planet. So, here are 5 reasons why half-degree rise in Earth’s temperature is a big deal.

1. It’s a Half-Degree Rise in Temperature Across the Globe

First, we’re not talking about a local half-degree rise in temperature lasting for an hour or two. We’re talking about a global rise lasting for a minimum of 30 years.

Let’s look at how the Earth’s temperature is actually measured. Every day, scientists take thousands of measurements from land instruments, weather buoys, weather ships and satellites. This data is then used to calculate average daily and annual temperatures. The annual average is then added to the last 29 years’ worth of data, in order to calculate a 30-year average global temperature. This is the official temperature of the surface of the planet. (See also: What is the Difference Between Climate & Weather?)

Okay, now imagine the amount of extra heat that’s needed to raise the temperature of the entire planet by half a degree and keep it raised for 30 years! Especially since at least 30 percent of that heat is neutralized by the ocean. (See our article: Effects of Global Warming on the Oceans.)

If you prefer, think of it as money. If everyone in the world was given half a dollar, they wouldn’t start dancing in the streets, would they? No. Because half a dollar is nothing. But multiply one half-dollar by the number of people in the world (7 billion) and you get $3.5 billion, which is a huge amount of money.

It’s the same with temperature. An extra half-degree temperature rise across the whole world is a huge amount of heat, and it can have huge effects.

The converse is true too. In the past, a one or two degree drop was all that was needed to plunge Planet Earth into the Little Ice Age. And a five-degree fall was sufficient to bury a large chunk of North America under a gigantic mass of ice 20,000 years ago. 1

2. It’s a Global Average

Second, it’s an average. Which means some locations could be constantly hotter (or occasionally very much hotter) and some colder. In other words, the “average” rise might be half a degree, but in location X for a period of Y days, it might be 20 degrees hotter. And that extra 20 degrees might have very serious consequences for the environment.

For example, the recent half-degree rise in temperature (which has happened over the past 40 years or so) is already threatening the survival of green sea turtles. This is because the temperature of the beach sand that female turtles nest in, affects the gender of their offspring. At sand temperatures of 31.1 degrees Celsius (88 degrees Fahrenheit), only females hatch, while at 27.8 degrees Celsius (82 degrees Fahrenheit) and below, only males hatch. 2

This example shows how sensitive living creatures are to variations in temperature. But the same applies to crops. A half-degree averaged out over the whole world can produce a hefty increase in some locations and at certain times, which could push many crops to the thermal limit for their species. (See also our article: Earth’s Climate System: How Does it Work?)

According to Dave Schimel, who supervises NASA’s Jet Propulsion Laboratory’s Carbon Cycle and Ecosystems group, heat can endanger agriculture even when crops don’t die. “If you get really high temperatures or very dry conditions during critical parts of the development of the crop, it produces essentially no grain.” It may not die Schimel says, “but it doesn’t grow seed. It doesn’t grow a corncob. And the actual food part of the crop is dramatically inhibited above critical temperatures.” [3] 3 (See also our article: Why is Soil So Important to the Planet?)

The point to remember is this: each time the Earth heats up an extra half-degree the biosphere suffers, but in a variety of different ways. The effects of global warming vary region by region. According to one report cited by the IPCC, some regions, like the Arctic, heat up two to three times faster, while the Mediterranean and Middle East regions could see a 9 percent drop in water supply at 1.5 degrees of warming but a 17 percent drop at 2 degrees.

3. A Half-Degree Rise in Temperature is a Possible Tipping Point

Before humans began burning vast amounts of fossil fuels, beginning in the 18th/19th century, Earth and its ecosphere were part of a balanced geobiological system with stable temperatures (over a few millennia at least), stable concentrations of greenhouse gases (like CO2 and water vapor), stable sea levels and a stable climate. The planet enjoyed the benefit of a stable greenhouse effect, and its protective ozone layer remained intact, as were its polar ice caps and tropical rainforests.

Since 1900 (and particularly since the 1970s), the effects of fossil fuels have become more and more apparent, as our emissions of carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) continue to rise.

Today, as the Earth’s carbon cycle becomes progressively more unstable, scientists are warning that we may be getting close to a number of climate tipping points, beyond which we will lose control of key ecological assets – such as polar ice, tropical rainforests, and the northern permafrost. Were this to happen it would almost certainly trigger a disastrous rise in sea levels, and the inundation of major coastal cities in both hemispheres.

Although most climate models remain optimistic that such a catastrophe won’t happen just yet, the half-degree difference between 1.5 and 2 degrees could still prove to be a tipping point for vulnerable species and habitats. For example, according to the Intergovernmental Panel on Climate Change, 2 degrees spells extinction for all coral reefs – the so-called “rainforests of the sea” – who occupy less than 0.1 percent of the world’s ocean area, yet are home to at least 25 percent of all marine species. 4

What’s a tipping point?

Climate tipping points are critical thresholds that, if reached, lead to a runaway process of further warming. Possible tipping points include: (a) the collapse of Thwaites Glacier in the Antarctic ice sheet, which could lead to a serious rise in sea levels; (b) the wholesale thawing of the Arctic permafrost, which could see a catastrophic rise in temperature; (c) the drying out of 25-30 percent of the Amazon Basin, which could result in the “savannization” of the Amazon rainforest; (d) the disappearance of coral reefs, which will imperil 25 percent of marine life with unknown consequences for the global food web. And more.

4. The Climate Damage is Already Visible

The damage of the last half-degree rise (from the 1970s onwards) is already visible.

  • The last five full years (2014-18) have been the hottest ever, with record-breaking heatwaves in Australia (Jan 2019), Europe (July and August), India/Pakistan (June 2019). 5
  • The most consecutive years with one or more Category 5 hurricanes in each, are the last four: 2016-2019. Six storms have reached that category during this period – Matthew, Irma, Maria, Michael, Dorian, and Lorenzo. 6

The extreme heat experienced by the planet was also evidenced, in the northern hemisphere, by an outbreak of arctic wildfires throughout the tinder-dry boreal forests and taiga of Canada, Alaska, Greenland and Russia, during the summer of 2019. This disaster was matched in the southern hemisphere by bushfires in Australia (2019/20) and throughout the Amazon Basin in Brazil. See also: Deforestation in the Amazon Rainforest.

Helicopter over Australian Fires
Australian Fires 2019 Image: State Government of Victoria

NOTE: To understand why governments are dragging their feet over climate action, see: Root Cause of Climate Change.

5. A Half-Degree Rise in Temperature Aggravates the Effects of Global Warming

As part of the historic Paris Climate Agreement, countries pledged to keep global warming below 2°C (3.6°F) while trying to limit temperature increase to 1.5C (2.7F). Based on a request by governments, the Intergovernmental Panel on Climate Change (IPCC) studied how a 1.5°C rise in temperature might differ in effect from a 2°C rise.

The results, published in the IPCC’s Special Report on Global Warming of 1.5°C (2018), highlighted the calamitous differences that half a degree can make on climate pattersns as well as the frequency of extreme weather events and their severity. Here is a short survey of their findings.

Extreme Heat

At warming of 1.5°C, the percentage of global population exposed to severe heat every 5 years, was projected at 14 percent. At 2°C warming this increased to 37 percent.

Sea-Ice Free Arctic

Ice-free summers in the Arctic are forecast to occur once every 100 years at warming of 1.5°C, to once every decade at 2°C.

Sea Level Rise (SLR)

The IPCC forecast sea level rise of 40cm (1.3 feet) (1.5°C) and 46cm (1.5 feet) (2°C). This means up to 10 million fewer people would be exposed to related risks, based on 2010 population levels. 7 In its special report Ocean and Cryosphere in a Changing Climate (SROCC) (2019) the IPCC upped this forecast by about 50 percent.

Meantime, other reports have revealed that the Greenland ice sheet is losing ice seven times faster today than during the 1990s. They also point to structural weaknesses in the glaciers and ice sheets of Antarctica, warning of possible sea level rises of between 1m and 2m by the end of this century. 8 9

NOTE: A global rise in sea level of 1 metre (3.3 feet), will almost completely inundate the coastal areas of Bangladesh, displacing up to 35 million inhabitants.

Loss of Wildlife Species

A half-degree rise in temperature can have a huge impact on small life forms. For example, pollinators, like bees and hoverflies that support and maintain terrestrial productivity, including agriculture for human food consumption, have much larger habitats at 1.5°C warming than at warming of 2°C.

Overall, the report studied 105,000 species of insects, plants and vertebrates. At 2°C warming, 18 percent of insects, 16 percent of plants and 8 percent of vertebrates are projected to lose more than half their habitats. With 1.5°C of warming, 6 percent of insects, 8 percent of plants and 4 percent of vertebrates will be affected.

Damage to Ecosystems

Ecosystems will be seriously affected by a half-degree rise in temperature. For example, at 2°C of warming, 13 percent of the Earth’s land area is expected to witness shifts in biomes (such as changing from rainforest to savanna, or from tundra to forest), or transformation. With 1.5°C of warming, this risk is lowered to 4 percent of Earth’s land area.

Thawing of Northern Permafrost

With 1.5°C of warming, some 4.8 million sq km of permafrost is forecast to thaw out by 2100. At 2°C of warming, this increases to 6.6 million sq km.

Loss of Corals

From 2016 to 2018, almost half the coral in the Australian Great Barrier Reef perished. 10 Between now and the end of the century all coral reefs are predicted to be wiped out if warming reaches 2°C. If warming is limited to 1.5°C, 10 percent of coral reefs are predicted to survive.

Loss of Fish

Loss of fishery productivity is predicted to be more pronounced with warming higher than 1.5°C. For example, one the report found that the global annual catch from marine fisheries decreased by 1.5 million tons under 1.5°C of warming, but grew to 3 million tons at 2°C of warming.

Water Shortages

Between 184 and 270 million more people are projected to be exposed to water shortages in 2050 at 2°C warming than at 1.5°C warming.

What Needs to Be Done to Limit Warming to 1.5°C Rather Than 2°C?

In order to limit climate change to either the 1.5°C or the 2°C limit, we need to impose significant cuts in our greenhouse gas emissions, which will necessitate dramatic reforms across all economies, industries and geographies. The IPCC lays out a 2-step process to implements these cuts: step 1 to be completed by 2030, step 2 by 2050 or 2075.

Graph showing how emissions trajectories affect the half degree rise in temperature.
Graph: Earth’s current trajectory (red) heading for a rise of 3.5 degrees, versus what needs to be done to limit temperature rises to 1.5 and 2 degrees. The half-degree rise in temperature makes a huge difference to the biosphere. See also: Earth’s Energy Balance.

Step 1. To limit warming to 1.5°C, we need to lower our annual emissions to an average of 25-30 billion tons of CO2 by 2030. Unfortunately, we are currently on track to pump out roughly 52-58 billion tons by 2030 – more than double this allowance. (See our article: What is the Emissions Gap?)

In terms of percentages, in order to keep warming to 1.5°C we need to reduce our annual emissions by about 40-50 percent (below 2010 levels) by 2030. To limit warming to 2°C, we need to reduce emissions by 20 percent.

Step 2. In order to reach 1.5°C we need to reduce our emissions to net-zero by 2030. In order to reach 2°C, the net-zero target must be met by 2075. 11

References

  1. “World of Change: Global Temperatures.” NASA, Jet Propulsion Laboratory. 2011. (1) []
  2. Jensen, Michael & Allen, Camryn & Eguchi, Tomoharu & P. Bell, Ian & L. LaCasella, Erin & A. Hilton, William & Hof, Christine & H. Dutton, Peter. (2018). Environmental Warming and Feminization of One of the Largest Sea Turtle Populations in the World. Current Biology. 28. 154-159.e4. (2) []
  3. Why a half-degree temperature rise is a big deal.” Bob Silberg. June 29, 2016. (3) []
  4. IPCC Special Report on Global Warming of 1.5°C (SR15) (4) []
  5. “India reels as summer temperatures touch 50°C”. BBC News. 3 June 2019. (5) []
  6. “Hurricane Dorian Becomes the 5th Atlantic Category 5 in 4 Years”. The Weather Channel. September 1, 2019. (6) []
  7. “Why does 1.5C warming goal matter to global poverty and health?” Reuters. Oct 8, 2018. (7) []
  8. Ice sheet contributions to future sea-level rise from structured expert judgment.” Jonathan L. Bamber, Michael Oppenheimer, Robert E. Kopp, Willy P. Aspinall, Roger M. Cooke. PNAS June 4, 2019 116 (23) 11195-11200; May 20, 2019. (8) []
  9. Evolving Understanding of Antarctic Ice-Sheet Physics and Ambiguity in Probabilistic Sea-Level Projections.” Robert E. Kopp, Robert M. DeConto, Daniel A. Bader, Carling C. Hay, Radley M. Horton, Scott Kulp, Michael Oppenheimer, David Pollard, Benjamin H. Strauss. Earth’s Future. 13 December 2017. (9) []
  10. “The Oceans We Know Won’t Survive Climate Change, Sea-level rise will become unmanageable, and life will flee the world’s tropical oceans, if carbon pollution keeps rising, a new report from the UN climate panel says.” Robinson Meyer. The Atlantic.com Sept 25, 2019. (10) []
  11. “Half a Degree and a World Apart: The Difference in Climate Impacts Between 1.5°C and 2°C of Warming.” World Resources Institute. Kelly Levin. October 07, 2018. (11) []
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