EmpHeir : 9 questions about climate change you were too embarrassed to ask

9 questions about climate change you were too embarrassed to ask

Basic answers to basic questions about the Paris climate agreement and global warming

President Donald Trump on Thursday made his final call to pull the United States out of the Paris climate agreement. The deal, joined by all but two countries (Syrian and Nicaragua), is a broad framework designed to nudge nations to prevent catastrophic climate change.

Climate change and global warming, not to mention the Paris agreement, are oft-misconstrued issues. Here are the most basic answers to basic questions about them.

1) What is the Paris climate agreement?

The deal was hammered out over weeks of tense negotiations in December 2015 and weighs in at 31 pages. What it does is actually pretty simple.

The backbone is the global target of keeping global average temperatures from rising 2°C (compared to temperatures pre-industrial revolution) by the end of the century. Beyond 2 degrees, we risk dramatically higher seas, changes in weather patterns, food and water crises, and an overall more hostile world.

Critics have argued that the 2-degree mark is arbitrary, or even too low, to make a difference. But it’s a starting point, a goal that, before Paris, the world was on a track to wildly miss.

It’s voluntary

To accomplish this 2-degree goal, the accord states that countries should strive to reach peak emissions “as soon as possible." (Currently, we’re on track to hit peak emissions around 2030 or later, which will likely be too late.)

But the agreement doesn’t detail exactly how these countries should do so. Instead it provides a framework for getting momentum going on greenhouse gas reduction, with some oversight and accountability. For the US, the pledge involves 26 to 28 percent reductions by 2025. (Under Trump’s current policies, that goal is impossible.)

195 countries have agreed to it. But there’s also no defined punishment for breaking it. The idea is to create a culture of accountability (and maybe some peer pressure) to get countries to step up their climate game.

In 2020, delegates are supposed to reconvene and provide updates about their emission pledges and report on how they’re becoming more aggressive on accomplishing the 2-degree goal.

It asks richer countries to help out poorer countries

There’s a fundamental inequality when it comes to global emissions. Rich countries have plundered and burned huge amounts of fossil fuels, and gotten rich from them. Poor countries seeking to grow their economies are now shunned from using the same fuels. Many low-lying poor countries also will be among the first to bear the worst impacts of climate change.

So as part of the Paris agreement, richer countries, like the US, are supposed to send $100 billion a year in aid by 2020 to the poorer countries. And that amount is set to increase over time. Again, like the other provisions of the agreement, this isn’t an absolute mandate.

The agreement matters because we absolutely need momentum on this issue

The Paris agreement is largely symbolic, and it will live on even though Trump is pulling the US out. But as Vox’s Jim Tankersley writes, “the accord will be weakened, and, much more importantly, so will the fragile international coalition” around climate change.

2) What is global warming?

The world is getting hotter, and humans are responsible. That's the short version.

When people say global warming, they're typically referring to the rise in average temperature of the Earth's climate system since the late 19th century. Temperatures over land and ocean have gone up 0.8° Celsius (1.4° Fahrenheit), on average, in that span:

NASA

Many people also use the term "climate change" to describe this rise in temperatures and the associated effects on the Earth's climate.

The consensus among climate scientists is that this temperature increase has been driven primarily by the extra greenhouse gases humans have put into the atmosphere since the Industrial Revolution. Greenhouse gases like carbon dioxide trap heat at the Earth's surface, preventing that heat from escaping back out into space too quickly. So when we burn coal or oil for energy or cut down forests and add even more carbon dioxide to the atmosphere, the planet warms up.

Global warming also refers to what scientists think will happen in the future if humans keep adding greenhouse gases to the atmosphere. A 2013 report from the Intergovernmental Panel on Climate Change projects that temperatures could rise at least 2°C (3.6°F) by the end of the century under many plausible scenarios — and possibly 4°C or more.

Many experts consider 2°C of warming to be unacceptably high, increasing the risk of deadly heat waves, droughts, flooding, and extinctions. Rising temperatures will drive up global sea levels as the world's glaciers and ice sheets melt. Further global warming could affect everything from our ability to grow food to the spread of disease.

Avoiding drastic global warming would likely require a complete overhaul of our energy system. Fossil fuels currently provide 87 percent of the world's energy. To zero out emissions this century, we'd have to replace most of that with low-carbon sources like wind, solar, nuclear, geothermal, or carbon capture.

That's a staggering task, and there are huge technological and political hurdles standing in the way. As such, the world's nations have been slow to act on global warming — it's a genuinely difficult issue to tackle, and efforts to revamp the energy system often encounter heavy opposition.

3) How do we know global warming is real?

The simplest way is through temperature measurements. Agencies in the United States and Europe have independently analyzed historical temperature data and reached the same conclusion: the Earth's average surface temperature has risen roughly 0.8° Celsius (1.4° Fahrenheit) since the early 20th century.

But that's not the only clue. Scientists have also noted that glaciers and ice sheets around the world are melting. Satellite observations since the 1970s have shown warming in the lower atmosphere. There's more heat in the ocean, causing water to expand and sea levels to rise. Plants are flowering earlier in many parts of the world. There's more humidity in the atmosphere. Here's a summary from the National Oceanic and Atmospheric Administration:

These are all signs that the Earth really is getting warmer — and that it's not just a glitch in the thermometers. That explains why climate scientists say things like, "Warming in the climate system is unequivocal." They're really confident about this one.

4) How do we know humans are causing global warming?

Climate scientists say they are 95 percent certain that human influence has been the dominant cause of global warming since 1950. They're about as sure of this as they are that cigarette smoke causes cancer.

Why are they so confident? In part because they have a good grasp on how greenhouse gases can warm the planet, in part because the theory fits the available evidence, and in part because alternate theories have been ruled out. Let's break it down in six steps:

1) Scientists have long known that greenhouse gases in the atmosphere — such as carbon dioxide, methane, or water vapor — absorb certain frequencies of infrared radiation and scatter them back toward the Earth. These gases essentially prevent heat from escaping too quickly back into space, trapping that radiation at the surface and keeping the planet warm.

2) Climate scientists also know that concentrations of greenhouse gases in the atmosphere have grown significantly since the Industrial Revolution. Carbon dioxide has risen 40 percent. Methane has risen 150 percent. Through some relatively straightforward chemistry, scientists can trace these increases to human activities like burning oil, gas, and coal.

3) So it stands to reason that more greenhouse gases would lead to more heat. And indeed, satellite measurements have shown that less infrared radiation is escaping out into space over time and instead returning to the Earth's surface. That's strong evidence that the greenhouse effect is increasing.

4) There are other human fingerprints that suggest increased greenhouse gases are warming the planet. For instance, back in the 1960s, simple climate models predicted that global warming caused by more carbon dioxide would lead to cooling in the upper atmosphere (because the heat is getting trapped at the surface). Later satellite measurements confirmed exactly that. Here are a few other similarpredictions that have also been confirmed.

5) Meanwhile, climate scientists have ruled out other explanations for the rise in average temperatures over the past century. To take one example: Solar activity can shift from year to year, affecting the Earth's climate. But satellite data shows that total solar irradiance has declined slightly in the past 35 years, even as the Earth has warmed.

6) More recent calculations have shown that it's impossible to explain the temperature rise we've seen in the past century without taking the increase in carbon dioxide and other greenhouse gases into account. Natural causes, like the sun or volcanoes, have an influence, but they're not sufficient by themselves.

Ultimately, the Intergovernmental Panel on Climate Change (IPCC) concluded that most of the warming since 1951 has been due to human activities. The Earth's climate can certainly fluctuate from year to year due to natural forces (including oscillations in the Pacific Ocean, such as El Niño). But greenhouse gases are driving the larger upward trend in temperatures.

More: Here's a chart breaking down all the different factors affecting the Earth's average temperature. And there's much more detail in the IPCC's report, particularly here and here.

5) How has global warming affected the world so far?

Here's a list of ongoing changes that climate scientists have concluded are likely linked to global warming, as detailed by the Intergovernmental Panel on Climate Change (IPCC) here and here.

Higher temperatures: Every continent has warmed substantially since the 1950s. There are more hot days and fewer cold days, on average, and the hot days are hotter.

Heavier storms: The world's atmosphere can hold more moisture as it warms. As a result, the overall number of heavier storms has likely increased since midcentury, particularly in North America and Europe (though there's plenty of regional variation).

Heat waves: Heat waves have likely become longer and more frequent around the world over the past 50 years, particularly in Europe, Asia, and Australia.

Shrinking sea ice: The extent of sea ice in the Arctic has shrunk since 1979, by between 3.5 percent and 4.1 percent per decade, on average. Summer sea ice has dwindled even more rapidly:

Shrinking glaciers: Glaciers around the world have, on average, been losing ice since the 1970s. In some areas, that is reducing the amount of available freshwater.

Sea-level rise: Global sea levels rose 25 centimeters (9.8 inches) in the 19th and 20th centuries, after 2,000 years of relatively little change. The pace of sea-level rise has continued to increase in recent decades. Sea-level rise is caused by both the thermal expansion of the oceans — as water warms up, it expands — and the melting of glaciers and ice sheets.

Food supply: A hotter climate can be both good for crops (it lengthens the growing season, and more carbon dioxide can increase photosynthesis) and bad for crops (excess heat can damage plants). The IPCC found that global warming was currently benefiting crops in some high-latitude areas, but that negative effects were becoming increasingly common worldwide.

Shifting species: Many land and marine species have had to shift their geographic ranges in response to warmer temperatures. So far, only a few extinctions have been linked to global warming, such as certain frog species in Central America.

Debated impacts

Here are a few other ways the Earth's climate has been changing — but scientists are still debating whether and how they're linked to global warming:

Droughts have become more frequent and more intense in some parts of the world — such as the American Southwest, Mediterranean Europe, and West Africa — though it's hard to identify a clear global trend. In other parts of the world, such as the midwestern United States and northwestern Australia, droughts appear to have become less frequent. There's still a fair bit of debate on how global warming has affected droughts so far.

Hurricanes have clearly become more intense in the North Atlantic Ocean since 1970, the IPCC says. But it's less clear whether global warming is driving this. And there doesn't yet seem to be any clear trend for tropical cyclones worldwide.

6) What impacts will global warming have in the future?

It depends on how much the planet actually heats up. The changes associated with 4° Celsius (or 7.2º Fahrenheit) of warming are expected to be more dramatic than the changes associated with 2°C of warming.

Here's a basic rundown of some big impacts we can expect if global warming continues, via the Intergovernmental Panel on Climate Change (here and here).

Hotter temperatures: If emissions keep rising unchecked, then global average surface temperatures will be at least 2ºC higher (3.6ºF) than pre-industrial levels by 2100 — and possibly 3ºC or 4ºC or more.

Higher sea-level rise: The expert consensus is that global sea levels will rise somewhere between 0.7 and 1.2 meters by the end of the century if global warming continues unchecked (that's between 2 and 4 feet). And that's only the average. In regions like the eastern United States, sea-level rise could be even higher.

Heat waves: A hotter planet will mean more frequent and severe heat waves.

Droughts and floods: Across the globe, wet seasons are expected to become wetter, and dry seasons drier. As the IPCC puts it, the world will see "more intense downpours, leading to more floods, yet longer dry periods between rain events, leading to more drought."

Hurricanes: It's not yet clear what impact global warming will have on tropical cyclones. The IPCC said it was likely that tropical cyclones would get stronger as the oceans heat up, with faster winds and heavier rainfall. But the overall number of hurricanes in many regions was likely to "either decrease or remain essentially unchanged."

Heavier storm surges: Higher sea levels will increase the risk of storm surgesand flooding when storms do hit.

Agriculture: In many parts of the world, the mix of increased heat and drought is expected to make food production more difficult. The IPCC concluded that global warming of 1°C or more could start hurting crop yields for wheat, corn, and rice by the 2030s, especially in the tropics. (This isn't uniform, however: some crops may benefit from mild warming, such as winter wheat in the United States.)

Extinctions: As the world warms, many plant and animal species will need to shift habitats at a rapid rate to maintain their current conditions. Some species will be able to keep up; others likely won't. The Great Barrier Reef, for instance, may not be able to recover from major recent bleaching events linked to climate change. The National Research Council has estimated that a mass extinction event "could conceivably occur before the year 2100."

Long-term changes: Most of the projected changes above will occur in the 21st century. But temperatures will keep rising after that if greenhouse gas levels aren't stabilized. That increases the risk of more drastic longer-term shifts. One example: if West Antarctica's ice sheet started crumbling, for instance, that could push sea levels up significantly. The National Research Council deemed many of these rapid climate surprises unlikely this century, but a real possibility farther into the future.

7) Is it “dangerous” to have more than 2°C of global warming?

Most of the world's nations have promised to avoid dangerous interference in the Earth's climate system. That's often taken to mean preventing global average temperatures from rising more than 2° Celsius (3.6° Fahrenheit) above pre-industrial levels. Temperatures have already risen 0.8°C so far.

This 2°C limit has a long, tangled history. By some accounts it was pushed by a German advisory panel back in the early 1990s, who argued that letting temperatures rise more than 2°C (3.6°F) would bring us outside the temperature range that allowed human civilization to flourish in the first place. Subsequent research detailed a range of adverse impacts that would occur if temperatures rose more than 2°C, from increased risks of severe weather to adverse impacts on agriculture.

Still, by its nature, the 2°C limit is arbitrary. Any single limit would be. Some scientists have noted that we could see a range of significant impacts long before we hit 2°C — coral reefs could start dying, or tiny island nations like Tuvalu could get swallowed by the rising seas. Conversely, other impacts, such as declining crop yields in the United States, might not happen until we go above the threshold. Deciding how to weigh all that is a political judgment as much as a scientific one.

For now, international climate negotiations tend to center around 2°C. At the Copenhagen climate talks in 2009, almost every nation in the world agreed to endorse 2°C as an upper limit for allowable global warming. The main dissenters (particularly those island nations) were arguing for an even lower limit.

8) What happens if the world heats up more drastically — say 4°C?

The risks of climate change would rise considerably if temperatures rose 4° Celsius (7.2° Fahrenheit) above pre-industrial levels — something that's possible if greenhouse gas emissions keep rising at their current rate.

The Intergovernmental Panel on Climate Change says 4°C of global warming could lead to "substantial species extinctions," "large risks to global and regional food security," and the risk of irreversibly destabilizing Greenland's massive ice sheet.

One huge concern is food production: a growing number of studies suggest it would become significantly more difficult for the world to grow food with 3°C or 4°C of global warming. Countries like Bangladesh, Egypt, Vietnam, and parts of Africa could see large tracts of farmland made unusable by rising seas.

And humans could struggle to adapt to these conditions. Many people might think the impacts of 4°C of warming will simply be twice as bad as those of 2°C. But as a 2013 World Bank report argued, that's not necessarily true. Impacts may interact with each other in unpredictable ways. Current agriculture models, for instance, don't have a good sense of what will happen to crops if increased heat waves, droughts, new pests and diseases, and other changes all start combining.

"[G]iven that uncertainty remains about the full nature and scale of impacts," the World Bank report said, "there is also no certainty that adaptation to a 4°C world is possible." Its conclusion was blunt: "The projected 4°C warming simply must not be allowed to occur."

9) How do we stop global warming?

The world's nations would need to cut their greenhouse gas emissions by a lot. And even that wouldn't stop all global warming.

For example, let's say we wanted to limit global warming to below 2°C. To do that, the IPCC has calculated that annual greenhouse gas emissions would need to drop at least 40 to 70 percent by midcentury.

Emissions would then have to keep falling until humans were hardly emitting any extra greenhouse gases by the end of the century. We'd also likely need to pull some carbon dioxide out of the atmosphere. The blue line below shows the path emissions would have to take to for a better-than-even chance of staying below 2°C:

By contrast, if emissions fall less sharply (the yellow line) or keep growing indefinitely (the red line), then the world would likely be on track for more warming — 3°C or 4°C or more.

Cutting emissions that sharply is a daunting task. Right now, the world gets 87 percent of its primary energy from fossil fuels: oil, gas, and coal. By contrast, just 13 percent of the world's primary energy is "low-carbon": a little bit of wind and solar power, some nuclear power plants, a bunch of hydroelectric dams. That's one reason why global emissions keep rising each year.

To stay below 2°C, that would all need to change radically. By 2050, the IPCC notes, the world would need to triple or even quadruple the share of clean energy it uses — and keep scaling it up thereafter. Second, we'd have to get dramatically more efficient at using energy in our homes, buildings, and cars. And stop cutting down forests. And reduce emissions from agriculture and from industrial processes like cement manufacturing.

The IPCC also notes that this task becomes even more difficult the longer we put it off, because carbon dioxide and other greenhouse gases will keep piling up in the atmosphere in the meantime, and the cuts necessary to stay below the 2°C limit become more severe.

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9 questions about climate change you were too embarrassed to ask