Half a degree can make a world of difference | Science News for Students

Half a degree can make a world of difference

Limiting global warming to 1.5 degrees Celsius versus 2 has big benefits, the IPCC says
Oct 18, 2018 — 6:30 am EST
an aerial image of the Maldives

Due to impending sea level rise, this island nation in the Indian Ocean — the Maldives — is among the most threatened anywhere by climate change.


The average temperature of the planet has been rising since the 1700s and its Industrial Revolution. Three years ago, in 2015, 195 nations signed onto what is known as the Paris Accord. In it, they agreed to curb greenhouse gas emissions to limit that warming by 2100. The goal is not to let it exceed 2 degrees Celsius (3.6 degrees Fahrenheit) over preindustrial times. But a new report says there may be big benefits to setting an even lower target.

If Earth warms by just 1.5 degrees C (2.7 degrees F), we would see fewer life-threatening extremes in heat, drought and precipitation. There also would be less sea level rise and fewer species would go extinct.

The new report comes from the Intergovernmental Panel on Climate Change, or IPCC. A summary of its full report came out October 8. It followed a weeklong meeting of scientists and policy makers in Incheon, South Korea. “This will be one of the most important meetings in the IPCC’s history,” said Hoesung Lee in his opening address. He spoke on October 1. He is a climate economist who works at Korea University in South Korea. He’s also the current IPCC chair.

For their report, scientists sifted through more than 6,000 papers. Those papers probed the impact of a global warming of 1.5 degrees. Sometimes the scientists even worked through the night, says Natalie Mahowald. She’s a climate scientist at Cornell University in Ithaca, N.Y. She’s also one of the report’s coauthors. The long hours were worth it. The report’s message is compelling and urgent, she says. “Such a small change in temperature will have big impacts on people.”

Curbs called for in the Paris Accord were designed to keep global warming to “well below” 2 degrees by 2100. Getting all the delegates to agree on that was hard. But many scientists have warned that this limit isn’t good enough. It won’t halt major environmental changes. And it could affect everything from sea level rise and water scarcity to habitat loss.

During the Paris talks, more than 100 nations called for a lower target — 1.5 degrees Celsius. Those nations included many of the countries at greatest risk from climate change. Examples are the Indian Ocean island nation of the Maldives and drought-stricken Angola in Africa.

Until very recently, scientists knew relatively little about how the risks from a 1.5-degree warming would differ from those due to a warming of 2 degrees. (Lee noted this in his October 1 address.) So the nations invited the IPCC to prepare a report to study that. Creating that report was a part of the decision to adopt the Paris Accord.

a photo of a conifer forest
Growing forests soak up lots of carbon dioxide and store the carbon in their trunks and roots. That's why preserving forests and creating new ones is one natural way to counter the rise in greenhouse gases.

1.5 degrees versus 2

The two warming targets would lead to very different changes. They are outlined in the new report. By reducing the warming to 1.5 degrees, people would experience fewer extremes in heat, rain and drought. A half a degree less warming also would shave an average of about 0.1 meter (4 inches) from sea-level rise by 2100. That would expose 10 million fewer people to related risks. These include flooding, infrastructure damage and saltwater breakthrough into freshwater resources.

There are great ice sheets in Antarctica and Greenland. Somewhere between 1.5 degrees and 2 degrees, these ice sheets could become very unstable. Their melting would further increase the potential for sea level rise. The Arctic Ocean is projected to be ice-free during the summer only once per century with just a 1.5-degree warming. That would increase to once a decade under the 2-degree scenario. 

There also would be benefits for plants and animals with a lower rise in temperature, the report notes. Less habitat would be lost for many species compared with 2 degrees of warming. Other climate risks to species also would be milder. Among those risks: forest fires and the spread of invasive species.

Many of the data analyzed in the report come from scientific papers published only in the past two years. They weren’t available when the Paris Accord was signed. The London-based website Carbon Brief published an interactive infographic on October 4. It summarizes the results of 70 such 1.5-degree studies. They show the impacts of warming targets on everything from sea level and heatwaves to hurricanes.

Not an easy path

Despite building a case for a lower-temperature target, the trick will be how to get there. In 2017, the Paris Accord faced a major setback. That’s when President Donald Trump announced that the United States would pull out of the agreement. The United States is a major source of the greenhouse gases that drive warming. That makes hopes for achieving a far more stringent target especially daunting.

Scientists have examined various possible paths to limit the environmental harm due to warming. But almost all such paths share one thing in common, says Zeke Hausfather. He’s a climate scientist with Carbon Brief. They overshoot the 1.5-degree limit somewhere around the year 2050. Later, he says, they bring temps “back down.”

To overshoot the mark by only a small amount, or not at all, requires cutting emissions of greenhouse gases by a lot. That pollution would have to drop by about 45 percent compared to emissions in 2010. And that has to be done fast — in 11 years, or by 2030.

The IPCC wants nations of the world to reach “net zero” emissions of greenhouse gases by around 2050. To do that, technologies must be in place that pull greenhouse gases out of waste streams or the atmosphere at rates equal to what goes in. The result: The atmosphere receives no increase in warming gases. Even better would be to remove more greenhouse gases than are entering the atmosphere. Then overall levels of warming could fall.

If the overall limit on warming is just “below 2 degrees Celsius,” emissions by 2030 would need to decline less than half as much as for the 1.5 degree limit. And they could reach net zero up to 25 years later — by about 2075.

If such early, deep cuts don't happen, “negative emissions” will be necessary.  Negative emissions are, essentially, a hoped-for reduction in emissions. And they might bring temperatures back down after overshooting the mark mid-century. But to remove enough CO2 from the atmosphere to reverse the greenhouse effect would require new technologies.

Those might include carbon capture and storage. Some such so-called "hacks" exist. Today, however, these technologies are too costly to be put in place widely. And reversing the effects of greenhouse warming is not easy. “By and large,” Hausfather says, there’s a rather straight relationship between warming and growing levels of carbon dioxide in the atmosphere. “But once you start sucking [carbon dioxide] out of the atmosphere, that linear relationship breaks.” The fall in emissions must be greater than the growth in pollution had been that led to those elevated temps in the first place.

It’s uncertain how — or if — society will be able to use the new report's findings to reshape the climate accords. During the 2015 Paris talks, a proposed 1.5-degree target was met with strong resistance from big polluter nations, particularly China. And President Trump’s administration has stated that the deep cuts needed in greenhouse-gas emissions are too costly and not yet ready to implement.

That is one reason the 1.5-degree goal may seem daunting. But this IPCC report — and the two years of intense research that led up to it — has also forced scientists to review some ideas about what’s possible, says Kaisa Kosonen. She works in Helsinki, Finland, as a climate policy adviser for Greenpeace International. She traveled to South Korea for the IPCC meeting. Investing in things such as better energy efficiency may one day lead to “results you didn’t know even existed,” she says. “I’ve been inspired by how much optimism there still is among scientists.”

Indeed, one of the key messages from the report is that holding warming to 1.5 degrees “is not impossible,” Mahowald says. “But it will require really ambitious efforts. And the sooner the better. We have to start cutting emissions now,” she says. “We have to be very ambitious on sustainable energy and sustainable agriculture.” What’s more, she notes, making this happen will require people to change their behaviors, too. These range from conserving energy to eating different foods.

But people also would face huge adjustments in a world that’s 2 degrees warmer, or even higher, she warns. So despite the challenges, Mahowald points out, “it still might be easier to reach 1.5 than to adapt to those higher temperatures.”

Correction: This article has been edited to note that the Maldives are in the Indian Ocean, not the Pacific.

Power Words

(more about Power Words)

accord     A formal agreement between two or more groups, usually nations.

agriculture     The growth of plants, animals or fungi for human needs, including food, fuel, chemicals and medicine.

Antarctica     A continent mostly covered in ice, which sits in the southernmost part of the world.

Arctic     A region that falls within the Arctic Circle. The edge of that circle is defined as the northernmost point at which the sun is visible on the northern winter solstice and the southernmost point at which the midnight sun can be seen on the northern summer solstice. The high Arctic is that most northerly third of this region. It’s a region dominated by snow cover much of the year.

atmosphere     The envelope of gases surrounding Earth or another planet.

average     (in science) A term for the arithmetic mean, which is the sum of a group of numbers that is then divided by the size of the group.

carbon     The chemical element having the atomic number 6. It is the physical basis of all life on Earth. Carbon exists freely as graphite and diamond. It is an important part of coal, limestone and petroleum, and is capable of self-bonding, chemically, to form an enormous number of chemically, biologically and commercially important molecules.

carbon dioxide (or CO2)     A colorless, odorless gas produced by all animals when the oxygen they inhale reacts with the carbon-rich foods that they’ve eaten. Carbon dioxide also is released when organic matter burns (including fossil fuels like oil or gas). Carbon dioxide acts as a greenhouse gas, trapping heat in Earth’s atmosphere. Plants convert carbon dioxide into oxygen during photosynthesis, the process they use to make their own food.

climate     The weather conditions that typically exist in one area, in general, or over a long period.

climate change     Long-term, significant change in the climate of Earth. It can happen naturally or in response to human activities, including the burning of fossil fuels and clearing of forests.

coauthor     One of a group (two or more people) who together had prepared a written work, such as a book, report or research paper. Not all coauthors may have contributed equally.

conservation     The act of preserving or protecting something. The focus of this work can range from art objects to endangered species and other aspects of the natural environment.

daunting     Something that is intimidating and which seems hard to imagine in detail so that you can figure out how to cope with, master or achieve it.

drought     An extended period of abnormally low rainfall; a shortage of water resulting from this.

factor     Something that plays a role in a particular condition or event; a contributor.

freshwater     A noun or adjective that describes bodies of water with very low concentrations of salt. It’s the type of water used for drinking and making up most inland lakes, ponds, rivers and streams, as well as groundwater.

global warming     The gradual increase in the overall temperature of Earth’s atmosphere due to the greenhouse effect. This effect is caused by increased levels of carbon dioxide, chlorofluorocarbons and other gases in the air, many of them released by human activity.

greenhouse     A light-filled structure, often with windows serving as walls and ceiling materials, in which plants are grown. It provides a controlled environment in which set amounts of water, humidity and nutrients can be applied — and pests can be prevented entry.

greenhouse effect     The warming of Earth’s atmosphere due to the buildup of heat-trapping gases, such as carbon dioxide and methane. Scientists refer to these pollutants as greenhouse gases. The greenhouse effect also can occur in smaller environments. For instance, when cars are left in the sun, the incoming sunlight turns to heat, becomes trapped inside and quickly can make the indoor temperature a health risk.

greenhouse gas     A gas that contributes to the greenhouse effect by absorbing heat. Carbon dioxide is one example of a greenhouse gas.

Greenland     The world’s largest island, Greenland sits between the Arctic Ocean and North Atlantic. Although it is technically part of North America (sitting just east of Northern Canada), Greenland has been linked more politically to Europe. Ice covers roughly 80 percent of Greenland. Indeed, the Greenland ice sheet is the world’s largest. If its frozen water were to melt, it could raise sea levels around the world by 6 meters (about 20 feet). Although this is the 12th biggest nation (based on surface area), Greenland averages the fewest people per square kilometer of its surface area.

habitat     The area or natural environment in which an animal or plant normally lives, such as a desert, coral reef or freshwater lake. A habitat can be home to thousands of different species.

ice sheet     A broad blanket of ice, often kilometers deep. Ice sheets currently cover most of Antarctica. An ice sheet also blankets most of Greenland. During the last glaciation, ice sheets also covered much of North America and Europe.

implement     To put in place and work well.

Industrial Revolution     A period of time around 1750 that was marked by new manufacturing processes and a switch from wood to coal and other fossil fuels as a main source of energy.

infographic     A chart, table, diagram, photos or drawings together with data that present information in a largely visual way. In these presentations, text is kept to a minimum.

infrastructure     The underlying structure of a system. The term usually refers to the basic physical structures and facilities on which a society depends. These include roads, bridges, sewers, drinking water supplies, electrical power grids and phone systems.

Intergovernmental Panel on Climate Change, or IPCC.     This international group keeps tabs on the newest published research on climate and on how ecosystems are responding to it. The United Nations Environment Programme and the World Meteorological Organization jointly created the IPCC in 1988. Their aim was to provide the world with a clear scientific view on the current state of knowledge in climate change and its potential environmental and social impacts.

invasive species     (also known as aliens) A species that is found living, and often thriving, in an ecosystem other than the one in which it evolved. Some invasive species were deliberately introduced to an environment, such as a prized flower, tree or shrub. Some entered an environment unintentionally, such as a fungus whose spores traveled between continents on the winds. Still others may have escaped from a controlled environment, such as an aquarium or laboratory, and begun growing in the wild. What all of these so-called invasives have in common is that their populations are becoming established in a new environment, often in the absence of natural factors that would control their spread. Invasive species can be plants, animals or disease-causing pathogens. Many have the potential to cause harm to wildlife, people or to a region’s economy.

policy     A plan, stated guidelines or agreed-upon rules of action to apply in certain specific circumstances. For instance, a school could have a policy on when to permit snow days or how many excused absences it would allow a student in a given year.

precipitation     (in meteorology) A term for water falling from the sky. It can be in any form, from rain and sleet to snow or hail.

preindustrial     An adjective that refers to the period before societies had begun to industrialize, using machines and fossil fuels to build products, often with assembly lines or big teams of workers. In the United States, that period began in the mid- to late-1700s.

risk     The chance or mathematical likelihood that some bad thing might happen. For instance, exposure to radiation poses a risk of cancer. Or the hazard — or peril — itself. (For instance: Among cancer risks that the people faced were radiation and drinking water tainted with arsenic.)

scenario     A possible (or likely) sequence of events and how they might play out.

sea     An ocean (or region that is part of an ocean). Unlike lakes and streams, seawater — or ocean water — is salty.

sea level     The overall level of the ocean over the entire globe when all tides and other short-term changes are averaged out.

society     An integrated group of people or animals that generally cooperate and support one another for the greater good of them all.

species     A group of similar organisms capable of producing offspring that can survive and reproduce.

sustainable     An adjective to describe the use of resources in a such a way that they will continue to be available long into the future.

threshold     A lower limit; or the lowest level at which something occurs.

viable     Able to live and survive. (in biology) Able to survive and/or live a normal lifespan. (in engineering) Something that should work or operate according to plan, as in a “viable concept.”


Report: Intergovernmental Panel on Climate Change. IPCC special report on global warming of 1.5º C, summary for policymakers. 48th Session of the IPCC, Incheon, South Korea, published online October 8, 2018.

Report: U.S. National Highway Traffic Safety Administration. Environmental impact statement for CAFE standards, 2017–2025, July 2018.