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Why Is 1.5 Degrees the Danger Line for Global Warming?
The number may look small, but 1.5 degrees of global warming has huge consequences for our climate and our planet.
WHY IS 1.5 DEGREES THE DANGER LINE FOR GLOBAL WARMING?
How a seemingly small change in average temperatures could trigger lasting changes for life as we know it.
March 18, 2019 SHARE
Honestly, it’s a great question.
“Why is holding global warming to 1.5 degrees Celsius such a big deal?”
Many people think of the hottest days of the summer where temperatures already hit 40 degrees (that’s 104 degrees Fahrenheit) where they live – or hotter. Another degree or two is a little bit more uncomfortable, sure, but hardly feels like the end of the world.
So how does global warming crossing the 1.5-degree line become – as one reader put it – “an extinction-level event”?
Spoiler alert: it’s not. At least not for humans. But it is right about the point that scientists project we’ll see some of the climate impacts we already see today begin to go from bad to outright terrifying. It’s about the point where we’ll likely see many natural systems begin to cross dangerous points of no return, triggering lasting changes and transforming life as we know it.
To put it another way, we want to do everything we can to keep warming below 1.5 degrees.
To understand why, read on.
Global Warming Is about Average Temperatures
When we talk about 1.5 degrees of warming, we’re talking about the increase in the Earth’s average temperature. We measure this increase from a baseline average temperature in the mid-to-late nineteenth century – when the Industrial Revolution swung into high gear and people began burning fossil fuels on an unprecedented level, jumpstarting climate change.
The important thing to understand is that global warming that comes from burning fossil fuels is not a uniform process. Due to a host of natural factors, some areas – like the poles – are warming much faster than others. So when we talk about preventing 1.5 degrees of global warming, we’re talking about preventing a 1.5 degree increase in the Earth’s average temperature. Some places have already crossed that line.
Temperatures Will Get Much Hotter than 1.5 Degrees
Global warming reaching 1.5 degrees doesn’t mean that average temperatures in some places won’t rise significantly beyond that number. Again – it’s just the global average.
Then there’s the fact that as average temperatures rise, spikes and heatwaves will go much, much higher than just 1.5 degrees.
As the Intergovernmental Panel on Climate Change (IPCC) – basically the gold standard for climate science – reported: “Several regional changes in climate are assessed to occur with global warming up to 1.5°C compared to pre- industrial levels, including warming of extreme temperatures in many regions.”
That’s rock-star-scientist-speak for: “If global warming reaches up to 1.5 degrees, the hottest of the hot temperatures will increase and many (more) places will get dangerously hot.”
We got a preview of what “extreme temperatures in many regions” looked like in 2018.
In Pakistan, a May heatwave took temperatures above 110 degrees Fahrenheit (43.3 degrees Celsius) and cost 65 lives in one city alone.
Europe also had a taste of the new normal last summer, with temperatures soaring above 115 degrees Fahrenheit (46 degrees Celsius) in Portugal. It wasn’t just Portugal either – the same heatwave roasted countries across the continent, breaking records and costing yet more lives.
All of which is to say, 1.5 degrees is not the limit of how much hotter things will get at some points throughout the year. Far from it.
The Climate Crisis Doesn’t Start at 1.5 Degrees – It’s Already Here
Another critical thing to understand about global warming is that it’s not the case that everything up to 1.49999 degrees is rainbows and unicorns and free ice cream for everyone. (But once we cross the 1.5 degrees-line, the Four Horsemen of the Apocalypse polish off their martinis, look at each other, and say, “It’s go time.”)
That’s because the climate crisis is already here. Today. Higher temperatures are already dragging out droughts and wiping out crops. Himalayan glaciers that provide water to some 240 million people are already melting. Storms like Hurricanes Harvey, Irma, and Marie are already getting stronger and more devastating thanks to climate change. The list goes on.
All of these impacts (and so many more) involve complex systems. Some overlap. Some don’t. But what they all have in common is heat. Heat is the factor that throws natural systems with their delicate checks and balances out of whack.
The (over)simple version is that the more heat added to the Earth’s climate system, the more out of balance natural systems get. The more out of balance natural systems get the more destruction and suffering we see. And it’s almost always poor families and people of color who suffer the most.
So where does the 1.5-degree number fit in?
Well, at about 1.5 degrees of global warming is right about where there’s enough heat to push many of the natural systems that sustain us past a dangerous turning point.
Think of 1.5 degrees not as an absolute line in the sand, but as a general indicator of where many climate impacts – on balance – go from destructive to catastrophic. It’s the sign on the door that leads to somewhere very dark indeed, somewhere no one wants to go.
A Degree of Concern: Why Global Temperatures Matter – Climate Change: Vital Signs of the Planet
In Part 1 of our feature, we examined some of the many reasons why Earth’s natural and human systems are sensitive to a warming climate. In Part 2, we’ll highlight some of the specific ways the IPCC Special Report projects our planet may change with another half-degree or full degree Celsius of warming.
Credits (left to right): Derivative of "Bleached coral, Acoropora sp." by Vardhan Pantankar/Wikimedia Commons used under CC BY-SA 4.0; USDA; NASA/John Sonntag
A Degree of Concern: Why Global Temperatures Matter
By Alan Buis
NASA's Global Climate Change Website
FEATURE | June 19, 2019
Part 2: Selected Findings of the IPCC Special Report on Global Warming
In part one of our feature, we examined some of the many reasons why Earth’s natural and human systems are sensitive to a warming climate. In part two, we’ll highlight some of the specific ways the IPCC special report projects our planet may change with another half-degree or full degree Celsius of warming.
Part 1 of this two-part series includes an interactive presentation of highlights from the Intergovernmental Panel on Climate Change special report showing how higher temperature thresholds will adversely impact increasingly larger percentages of life on Earth, with significant variations by region, ecosystem and species.
Each of the following selected projections are from the IPCC special report. In most instances, climate-related risks for natural and human systems were found to be higher, often significantly so, under the hotter temperature threshold. The degree of these risks depends on many factors, such as the rate, duration and magnitude of warming; geographic location; levels of development and vulnerability; and on how humans respond through adaptation and mitigation options. Some regions, such as small island states, will experience multiple climate-related risks that compound upon each other.A key point of the special report is there is no single 1.5-degree warmer world.
The impacts of climate change haven’t been spread evenly around our planet and they won’t be in the future, either. Temperatures increase at different speeds everywhere, with warming generally higher over land areas than oceans. The strongest warming is happening in the Arctic during its cool seasons, and in Earth’s mid-latitude regions during the warm season.
Temperature change is not uniform across the globe. Projected changes are shown for the average temperature of the annual hottest day (top) and the annual coldest night (bottom) with 1.5 degrees Celsius of global warming (left) and 2 degrees Celsius of global warming (right) compared to pre-industrial levels. Credit: FAQ 3.1, Figure 1 from the Intergovernmental Panel on Climate Change Special Report on Global Warming of 1.5º Celsius (2.7º Fahrenheit). › Larger view
In many regions, warming has already surpassed 1.5 degrees Celsius above pre-industrial levels. More than one-fifth of all humans live in regions that have already seen warming greater than 1.5 degrees Celsius in at least one season. Climate-related risks were found to be generally higher at lower latitudes and for disadvantaged people and communities.
Temperature ExtremesWarm — According to the report, extreme temperatures on land are projected to warm more than the global average surface temperature, with substantial differences from place to place.
Figure 3.4 | Projected changes in extremes at 1.5 degrees Celsius (left) and 2 degrees Celsius (middle) of global warming compared to the pre-industrial period (1861–1880), and the difference between 1.5 degrees Celsius and 2 degrees Celsius of global warming (right). Temperature of annual hottest day (maximum temperature), TXx (top), and temperature of annual coldest night (minimum temperature), TNn (middle), and annual maximum 5-day precipitation, Rx5day (bottom). Credit: Figure 3.4 from the Intergovernmental Panel on Climate Change Special Report on Global Warming of 1.5º Celsius (2.7º Fahrenheit). › Larger view
Most land regions will see more hot days, especially in the tropics. At 1.5 degrees Celsius warming, about 14 percent of Earth’s population will be exposed to severe heatwaves at least once every five years, while at 2 degrees warming that number jumps to 37 percent. Extreme heatwaves will become widespread at 1.5 degrees Celsius warming.
Extreme heatwaves, like the one that affected Europe in the summer of 2006, are projected to become widespread at 1.5 degrees Celsius warming. This map, derived from NASA MODIS Terra satellite data, depicts the July 2006 land surface temperature anomaly with regard to the period from 2000-2012. Credit: Giorgiogp2 [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]
Limiting warming to 1.5 degrees Celsius would reduce the number of people frequently exposed to extreme heatwaves by about 420 million, with about 65 million fewer people exposed to exceptional heatwaves.
At Earth’s mid-latitudes, the hottest days will be up to 3 degrees Celsius (5.4 degrees Fahrenheit) hotter at 1.5 degrees Celsius warming and up to 4 degrees Celsius (7.2 degrees Fahrenheit) warmer at 2 degrees Celsius warming. The warmest extreme temperatures will be in Central and Eastern North America, Central and Southern Europe, the Mediterranean (including Southern Europe, Northern Africa and the near-East), Western and Central Asia and Southern Africa. Longer warm spells will affect many densely populated regions. At warming above 1.5 degrees Celsius, twice as many megacities as today are likely to become heat stressed, potentially exposing 350 million more people by 2050.
How the climate would change if the world warms over 1.5 degrees : NPR
Despite new pledges to cut emissions, the world is not on track to hit a key climate change target of limiting warming. Scientists warn a planet that heats up more than that will look very different.
This is what the world looks like if we pass the crucial 1.5-degree climate threshold
November 8, 20215:15 AM ET
LAUREN SOMMER Twitter
A kayaker paddles down an interstate in Pennsylvania after flooding from Hurricane Ida earlier this year. Several dozen people died, some in cars and basement apartments, during extreme flash flooding.
Branden Eastwood/AFP via Getty Images
There's one number heard more than any other from the podiums at the United Nations climate summit in Glasgow, Scotland: 1.5 degrees Celsius.
That's the global climate change goal world leaders agreed to strive for. By limiting the planet's warming to 1.5 degrees Celsius, or 2.7 degrees Fahrenheit, by 2100, the hope is to stave off severe climate disruptions that could exacerbate hunger, conflict and drought worldwide.
The 1.5 degree target has long been championed by developing nations, where millions of people are among the most vulnerable to climate change. At the 2015 Paris climate negotiations, they pushed industrialized countries to improve on the 2 degree Celsius goal held at the time, since wealthier nations are responsible for most greenhouse gas emissions since the Industrial Revolution.
At the climate negotiations now underway, nations are touting new commitments to cut their heat-trapping emissions by switching to clean energy and reducing deforestation. India is pledging, for the first time, to be carbon neutral by 2070. More than 100 countries, including the United States, joined a global pact to cut methane, a potent greenhouse gas.
Still, added together, the recent pledges don't go far enough. Even with more ambitious emissions cuts from some countries, warming is still on track for more than 2 degrees Celsius (or 3.6 Fahrenheit) by the end of the century. The Earth is already 1.1 degrees Celsius hotter than it was 150 years ago.
Though a half-degree Celsius difference in temperature increase might seem inconsequential, the difference for life on Earth could be huge. Here's what scientists expect, if average global temperatures exceed 1.5 degree Celsius warming by 2100.
Coral reefs face almost complete die-off
Off the coast of Australia, the Great Barrier Reef is known for being large enough to be seen from space. It's the size of Germany — a biodiversity hotspot that was once thought to be too big to fail. But over the last few decades, marine biologists like Ove Hoegh-Guldberg of the University of Queensland have watched its rapid decline.
"My career is one of going from a period of when it was wonderful and abundant to now, staring down the barrel," Hoegh-Guldberg says. "We're staring down the barrel of something really horrific."
Corals, like these in French Polynesia, turn white as they bleach during marine heat waves. In a world with 2 degrees Celsius of warming, 99 percent of corals are expected to die off.
Alexis Rosenfeld/Getty Images
Oceans are warming along with the atmosphere, since they absorb much of the excess heat from climate change. Repeated marine heat waves over the last five years have turned much of the Great Barrier Reef a ghostly white color. When temperatures rise, corals expel the microscopic algae inside them, losing their food source in the process. Sometimes the corals can recover, but increasingly, they're dying off.
"Something around 50% of the shallow water corals were killed literally over a couple of months, in some cases over a couple of weeks," Hoegh-Guldberg says. "If you extend that out into the future, we'll get to a point where the damage overwhelms the ability of corals to bounce back."
Marine heat waves have already doubled in number since 1980 and are expected to become more intense as temperatures rise. At 1.5 degrees Celsius, it's likely that 70 to 90% of coral reefs will die off worldwide. At 2 degrees Celsius of warming, 99% are lost.
"If we delay even a year or two more, we really are going down a pathway where there will be no return," Hoegh-Guldberg says. "We need to act and we need to act decisively, without question and solve this problem."
'Unheard-of' storms become more common
Water has taken a heavy toll in 2021, all over the world. In September, the remnants of Hurricane Ida swept into the Northeast, killing more than 50 people in New Jersey, New York and other states. Many drowned in cars and basement apartments, overwhelmed by rushing water. In August, two dozen people died when heavy rains caused flash flooding in Tennessee.
Scientists warn that a hotter atmosphere is a wetter atmosphere. Warmer air can hold more water, helping produce more intense rainfall and stronger storms. With ocean temperatures higher in the Gulf of Mexico, hurricanes are intensifying at a more rapid rate.
As the climate warms, storms once thought to be extremely rare are expected to become more frequent. And the chances don't just go up a little bit.
"When we think about the odds, it's not simply that twice the warming gives you twice the odds," says Gabriel Vecchi, professor of geosciences at Princeton University. "If we go on a path to 3 degree warming, more and more things that are unheard of or have been unheard of will become relatively commonplace."