Global warming and climate change
by Chris Woodford. Last updated: June 21, 2013.
It's hard to think of many things powerful enough to disrupt life across our entire planet. Huge natural disasters—like earthquakes, erupting volcanoes, or tsunamis (freak tidal waves)—can affect many thousands of people, but their impacts are usually confined to just one region of the world. Terrorist attacks cause worldwide panic and horror, but their effects are usually quite localized. Catastrophic nuclear explosions, like the one that happened at the Chernobyl nuclear plant in the Ukraine in 1986, can spread "fallout" (toxic radioactive debris) across an entire continent—but even they do not affect the whole Earth.
Global warming, which is a gradual rising of Earth's temperature, is different from all these, representing a scale of threat greater than anything humans have faced in recent history. Unless we tackle the problem soon, it could transform the planet we life on, making the climate (Earth's weather patterns) much more erratic, forcing many species into extinction, and making life much harder—especially for people in developing countries.
Photo: Is global warming too hot an issue for politicians to handle? This falsely colored image shows the amount of heat leaving different parts of Earth, as measured by NASA's Terra spacecraft. The blue areas are coldest, where thick clouds prevent heat from escaping. In the yellow areas, there is little cloud cover so the heat escaping is at a maximum. The red areas represent mostly ocean, where heat loss is midway between these two extremes. Picture courtesy of NASA Marshall Space Flight Center (NASA-MSFC).
What is global warming?
Imagine you live in a timber shack in Alaska. It's chilly up there, so you build yourself a huge log fire and pile on all the wood you can find. To start with, the fire seems a great idea—especially since it's so cold outside. The shack warms up slowly, but predictably, and it's soon pretty cosy. Since the shack is much warmer than the atmosphere and ground that surround it, it loses heat quite quickly. If the fire supplies heat at the same rate as the shack loses it, the shack stays at roughly the same temperature. But if you make the fire too big, the shack will get hotter... and hotter... and hotter. Before long, you'll start feeling uncomfortable. You might wish you'd never made the fire so big in the first place. But once it's burning, there's nothing you can do to stop it. The shack will keep getting hotter long after you stop piling wood on the fire.
Global warming is working a bit like this. Thanks to a variety of
things that people do, Earth is getting slightly warmer year by year.
It's not really warming up noticeably—at least not in the short term.
In fact, since 1900, the whole planet has warmed up only
by around 0.8 degrees Celsius. By the end of the 21st century,
however, global warming is likely to cause an increase in Earth's temperature of around
2–5 degrees Celsius. There is a 75 percent chance of a 2–3 degree
warming and a 50 percent chance of a 5 degree warming, and scientists
agree that the warming is most likely to be around 3 degrees.
Now even a 5-degree warming might not sound like
much to worry about, but 5 degrees is roughly how much difference there
is between the world as it is today and as it was during the last Ice
In other words, when we came out of the Ice Age, the planet
warmed by 5 degrees over about 5000 years.
Modern climate change threatens to produce the same amount of a warming
in as little as a century!
Once something as big as a planet starts to warm up, it's very hard to
slow down the
process—and almost impossible to stop it completely. Global warming
means Big Trouble.
Photo: Getting warmer? A NASA map of global sea surface temperatures produced using infrared measurements taken by a satellite in space. Red and yellow areas are hottest, green and blue are coldest. Picture courtesy of Great Images in NASA.
What causes global warming?
Global warming is caused by a phenomenon known as the greenhouse effect. A greenhouse (or glasshouse) is good for growing things because it traps heat inside and stays hotter than the atmosphere around it.
The natural greenhouse effect
Earth's atmosphere behaves like a gigantic greenhouse, though it traps heat a different way. Gases high in the atmosphere, such as carbon dioxide and methane, behave like a giant piece of curved glass wrapped right round the planet. The Sun's rays (mostly visible light and short-wavelength, high-energy ultraviolet radiation) pass straight through this greenhouse gas and warm up Earth. The warming planet gives off heat energy (longer wavelength infrared radiation), which radiates out toward space. Some of this outgoing radiation does not pass through the atmosphere, but is reflected back down to Earth, effectively trapping heat and keeping the planet about 33 degrees hotter than it would otherwise be. This is called the natural greenhouse effect and it's a good thing. Without it, Earth would be much too cold to support the huge diversity of life that it does.
Artwork: The Greenhouse Effect: 1) When the Sun's radiation enters our atmosphere, it heats our planet. 2) Like all hot objects, Earth gives off some of its heat as radiation of its own. Some of this radiation passes straight through the atmosphere and disappears off into space. 3) However, some is reflected back again by the "blanket" of greenhouse gases in the atmosphere. The more greenhouse gases there are, the more heat is trapped and the hotter Earth becomes.
The enhanced greenhouse effect
The greenhouse effect would be nothing to worry about were it not for one important thing. Since the Industrial Revolution in the 18th and 19th centuries (when coal-burning steam engines were first used on a large scale), humans have been using energy in far greater quantities. Car engines, for example, which were invented in the mid-19th century, work by burning gasoline (petrol) with oxygen from the air to make heat in a chemical reaction called combustion. As a byproduct, combustion gives off (or "emits") invisible carbon dioxide gas (the gas our bodies breathe out). In a similar way, power plants use combustion to make our electricity—by burning fuels like coal, gas, and oil—so they give off carbon dioxide too. Most of the energy people use is made by burning these so-called fossil fuels—producing huge clouds of carbon dioxide, which are known as carbon dioxide emissions. The carbon dioxide drifts up into the atmosphere and makes Earth's greenhouse gas just a little thicker. This is called the enhanced greenhouse effect. As a result, more of the Sun's heat gets trapped inside the atmosphere and the planet warms up. To summarize: burning fossil fuels give off carbon dioxide, which increases the greenhouse effect and heats the planet—the process we call global warming. This is often described as an anthropogenic process, which simply means "humans caused it." Although a small minority of people dispute this, the overwhelming majority of the world's climate scientists believe that global warming is "very likely" caused by human greenhouse gas emissions.
Is global warming getting worse?
Thanks to all the fossil fuels we burn, there is now more carbon dioxide in the atmosphere than at any time in the last 420,000 years. However, the actual amount of carbon dioxide in the atmosphere is still relatively small. Before industrial times, it was about 280 parts per million (ppm). Today, it's around 380 ppm. That means if you had a chunk of atmosphere about as big as your bedroom, all the carbon dioxide in it would take up 380 millionths of the space—or roughly half the volume of a shoebox. Doesn't sound much to worry about, does it? But the important thing is that the amount of carbon dioxide is rising: in the last 150 years or so, humans have increased the carbon dioxide in the atmosphere by around a third—and that's a very big change for something as finely balanced as our planet.
Most people have no idea how much carbon dioxide they generate each
day. The Carbon
web page gives you some idea what your personal carbon dioxide emissions
The United States produces roughly 20 tonnes of
carbon dioxide for each one of its citizens each year. Or to put it
another way, that's enough carbon dioxide to cover the entire land
surface of the United States 30 cm (1 ft) deep. Most of this gas winds
up in the atmosphere and contributes to global warming.
The problem is getting worse all the time. Currently, 80 percent of our energy comes from fossil fuels. According to the International Energy Institute, this could increase to 90 percent by 2020. And the amount of energy people use is increasing too, not least because developing countries such as China and India are becoming more affluent. According to the US government's Energy Information Administration, world energy consumption will increase by 71 percent between 2003 and 2030. In summary, if things continue as they are, we'll soon be using nearly twice as much energy and getting even more of it from fossil fuels. Without drastic action, the amount of carbon dioxide in the atmosphere will continue to increase—and Earth will continue to heat up. In other words, global warming will get worse.
Photo: Warming Island: These photos taken from the USGS Landsat satellite in 1985, 2002, and 2005 show how a new island has appeared in Greenland following the melting of an arctic glacier. Melting glaciers are one indication that the world is warming up. Picture courtesy of US Geological Survey (USGS): Landsat.
What is climate change?
Climate is the pattern of weather in a particular place: how much sunlight and rainfall it gets, how windy it is, and so on. The world's weather is entirely powered by the Sun. Since Earth rotates on a tilted axis, different parts of our planet are heated by different amounts at different times of year, making some regions hotter than others and causing the seasons. The temperature variations between one part of the world and another cause differences in air pressure, producing winds, storms, and even hurricanes. The Sun's heat also warms the seas unevenly, driving ocean currents—which, in some ways, are like underwater winds—from one place to another. Links between the atmosphere and the oceans can produce complex weather patterns such as El-Niño—a kind of abnormal and erratic weather that happens every few years in the Pacific.
Scientists believe that greater amounts of carbon dioxide in the atmosphere, and hotter temperatures on Earth, will significantly change the climate across the whole planet. This climate change is already beginning to happen in parts of the world. If you live in a chilly place like Alaska or Greenland, you might think a bit of global warming sounds like a great idea. But climate change doesn't necessarily mean things will get hotter. Some places will be hotter some of the time, but most places will simply see more erratic and extreme weather. That could mean heavier rainfall on occasions, more snow in some places, longer periods of drought, more storms and hurricanes, and more frequent heatwaves.
Photo: Will global warming bring more hurricanes?. Picture courtesy of NOAA Photo Library, NOAA Central Library; OAR/ERL/National Severe Storms Laboratory (NSSL).
Climate change is nothing new. Earth's climate has been changing regularly for hundreds of millions of years, sometimes getting colder and sometimes warmer. Everyone knows about Ice Ages—those periods of history when Earth was far colder than it is now. The climate change people talk about today seems to be different. Most scientists believe it is caused by systematic global warming, itself caused by a gradual increase in fossil fuels. Whereas traditional climate change makes Earth as a whole either hotter or cooler, modern climate change is going to make the climate much more erratic—hotter in some places, cooler in others; drier in some places; wetter elsewhere. In a nutshell, climate change means the type of weather we experience will change—perhaps quite dramatically in some places—as the years go by.
Is climate change really happening?
Scientists have been debating this question for about 20 years now.
So what's the official position? According to the Intergovernmental Panel on Climate Change (IPCC), which summarizes key scientific
research on the issue carried out by thousands of scientists, greenhouse gases have "increased markedly as a result of human activities since 1750 and now far exceed pre-industrial values" and "most of the observed increase in global average temperatures since the mid-20th century is very likely due to [this]... It is likely that there has been significant anthropogenic [human-caused] warming over the past 50 years averaged over each continent (except Antarctica)." As one of the world's leading climate scientists, Stefan Rahmstorf of
Potsdam Institute for Climate Impact Research, puts it, the current
scientific view of global warming is: "based on decades of research and
thousands of studies. The extraordinary consensus reached is seen in
the statements of many international and national professional bodies
which have extensively and critically assessed the scientific evidence."
Prof. Rahmstorf has compiled a very useful climate change fact sheet (PDF format) summarizing the evidence in a couple of pages. The BBC has produced a handy website on global warming with a section called Evidence, which contains charts showing climate trends over the last few years. Browse those charts and you will see that Earth's temperature has increased systematically over the last century, sea levels have risen significantly, and carbon dioxide emissions from burning fossil fuels have increased almost exponentially. Most climate scientists believe these things are connected: they consider that the burning fuels cause the carbon dioxide emissions, which make the temperature increase, which causes the sea levels to rise. (If you want to see lots more charts, take a look at the official IPCC reports website.)
Since records of the weather date back only a hundred years or so, how can scientists confidently make claims that the climate has been changing over a much longer period? In turns out that Earth keeps a natural record of its own climate in many surprising ways. For example, as ice has formed year upon year at the poles, old ice has been buried underneath with bubbles of air trapped inside it. The bubbles act as a record of what the air was like on Earth when the ice formed—and thus what the climate was like in years gone by. Using drills, scientists can extract ice cores (long thin pipes full of ice), study the air bubbles at different depths, and calculate how much carbon dioxide they contain. If they figure out how old the ice is, they can use an ice core as a kind of graph of how carbon dioxide has changed over time. Scientists can also study changes in the climate using ocean sediments, samples of buried pollen, and other, once-living matter. Research like this can tell us what the climate was like hundreds of thousands of years ago.
Although most scientists believe in global warming, it's important to note that a very small minority do not. Most agree that Earth is warming but not that fossil-fuel burning and human carbon dioxide emissions are responsible. The "climate-change skeptics" argue that increases in Earth's temperature are either not happening at all or may be caused by other things, including natural variations in the climate that have been happening for millennia. In recent years, however, fewer and fewer scientists have dissented from the widely held position that global warming and climate change are really happening. People could still be wrong about global warming—but that's becoming increasingly unlikely.