Introduction to Climate Change

The basics of global warming explained.

Weather Underground highly recommends The Rough Guide to Climate Change, Second Edition by Robert Henson and published by Rough Guides. If you are looking for a readable and thorough coverage of all the important facets of climate change, this is the book for you. We reprint the first chapter here.

Climate Change: A Primer

Key questions and answers

Before exploring the various aspects of climate change in depth, let's quickly answer some of the most frequently asked questions about the issue. The following fifteen pages will bring you up to speed with the current situation and the future outlook. For more information on each topic, follow the reference to the relevant chapter later in the book.

The big picture

Is the planet really warming up?

In a word, yes. Independent teams of scientists have laboriously combed through more than a century's worth of temperature records (in the case of England, closer to 300 years' worth). These analyses all point to a rise of more than 0.7°C (1.3°F) in the average surface air temperature of Earth over the last century (see the graph on the inside front cover). The chapter Keeping Track (p.171) explains how this average is calculated. The chart overleaf shows how warming since the 1970s has played out regionally.

In recent years global temperatures have spiked dramatically, reaching a new high in 1998. An intense El Niño (see p. 118) early that year clearly played a role in the astounding warmth, but things haven't exactly chilled down since then. The first six years of the twenty-first century, along with 1998, were the hottest on record – and quite possibly warmer than any others in the past millennium (see p. 220).

Apart from what temperatures tell us, there's also a wealth of circumstantial evidence to bolster the case that Earth as a whole is warming up.

  • Ice on land and at sea is melting dramatically in many areas outside of interior Antarctica and Greenland. Montana's Glacier National Park is expected to lose its glaciers by 2030. Arctic sea ice has lost nearly half its average summer thickness since 1950, and by mid-century the ice may disappear completely each summer, perhaps for the first time in more than a million years. The warmth is already heating up international face-offs over shipping, fishing and oil-drilling rights in parts of the Arctic once written off as inaccessible.
  • The growing season has lengthened across much of the Northern Hemisphere. The most common species of Japan's famed sakura (cherry blossoms) now blooms five days earlier on average in Tokyo than it did fifty years ago. At some higher latitudes, the growing season is now more than two weeks longer than it was in the 1950s – hardly a crisis in itself, but a sign that temperatures are on the increase.
  • Mosquitoes, birds and other creatures are being pushed into new territories, driven to higher altitudes and latitudes by increasing warmth. The range of twelve bird species in Britain shifted north in the 1980s and 1990s by an average of 19km (12 miles). And Inuits in the Canadian Arctic report the arrival over the last few years of barn swallows, robins, black flies and other previously unseen species. (As we'll see later, however, not all fauna will migrate so successfully.)

But don't many experts claim that the science is uncertain?

There is plenty of uncertainty about details in the global-warming picture: exactly how much it will warm, the locations where rainfall will increase or decrease, and so forth. Some of this uncertainty is due to the complexity of the processes involved, and some of it is simply because we don't know how individuals, corporations and governments will change their greenhouse emissions over time. But there's near-unanimous agreement that global climate is already changing and that fossil fuels are at least partly to blame.

The uncertainty that does exist has been played both ways in the political realm. Sceptics use it to argue for postponing action, while others point out that many facets of life require acting in the face of uncertainty (buying insurance against health or fire risks, for example).

Climate change or global warming?

The phrases that describe climate in transition have a history of their own. Early in the twentieth century, researchers preferred climatic change or climate change when writing about events such as ice ages. Both terms are nicely open-ended and still used often. They can describe past, present or future shifts – both natural and human-produced – on global, regional or local scales.

Once scientists began to recognize the specific global risk from human-produced greenhouse gases, they needed a term to describe it. In 1975 Wallace Broecker, of New York's Lamont-Doherty Earth Observatory, published a breakthrough paper in the journal Science entitled, "Climatic Change: Are We on the Brink of a Pronounced Global Warming?" By the early 1980s the phrase global warming – without the "a" in front – was gaining currency among scientists. Meanwhile, the term global change emerged as a way to embrace all modes of large-scale human tampering with the planet. When 1988's watershed events arrived (see p. 250), the global-warming label broke into headlines worldwide and became standard shorthand among media and the public.

Of course, the planet as a whole is warming, but many scientists avoid that term, preferring 'global change' or more specifically global climate change. One of their concerns is that global warming could be interpreted as a uniform effect – an equal warming everywhere on the planet – whereas in fact a few regions may cool slightly, even as Earth, on average, warms up.

Politicians hoping to downplay the reality of global warming gravitate towards 'climate change' for entirely different reasons. US political pollster and consultant Frank Luntz has reportedly advised clients that 'climate change' sounds less frightening to the lay ear than 'global warming'. Scary or not, a number of other surveys support the idea that 'global warming' gets people's attention more quickly than the less ominous (though more comprehensive) 'climate change.' And a few activists and scientists, including the Gaia theorist James Lovelock, now favour global heating – a phrase that implies humans are involved in what's happening.

Is a small temperature rise such a big deal?

While a degree or so of warming may not sound like such a big deal, the rise has been steeper in certain locations, including the Arctic, where small changes can become amplified into bigger ones (see p. 75). The warming also serves as a base from which heat waves become that much worse – especially in big cities, where the heat-island effect comes into play. Like a thermodynamic echo chamber, the concrete canyons and oceans of pavement in a large urban area heat up more readily than a field or forest, and they keep cities warmer at night. During the most intense hot spells of summer, cities can be downright deadly, as evidenced by the hundreds who perished in Chicago in 1995 and the thousands who died in Paris in 2003 (see p. 47).

How could humans change the whole world's climate?

By adding enormous quantities of carbon dioxide and other greenhouse gases to the atmosphere over the last 150 years. As their name implies, these gases warm the atmosphere, though not literally in the same way a greenhouse does. The gases absorb heat that's radiated by Earth, but they release only part of that heat to space, which results in a warmer atmosphere (see p. 24).

The amount of greenhouse gas we add is staggering – in carbon dioxide alone, the total is more than thirty billion metric tonnes per year, which is more than four metric tonnes per person per year. And that gas goes into an atmosphere that's remarkably shallow. If you picture Earth as a soccer ball, the bulk of the atmosphere would be no thicker than a sheet of paper wrapped around that ball.

Even with these facts in mind, there's something inherently astounding about the idea that a few gases in the air could wreak havoc around the world. However, consider this: the eruption of a single major volcano – such as Krakatoa in 1883 – can throw enough material into the atmosphere to cool global climate by more than 1°C (1.8°F) for over a year. From that perspective, it's not so hard to understand how the millions of engines and furnaces spewing out greenhouse gases each day across the planet, year after year, could have a significant effect on climate. (If automobiles spat out chunks of charcoal every few blocks in proportion to the invisible carbon dioxide they emit, the impact would be more obvious.) Yet many people respond to the threat of global warming with an intuitive, almost instinctive denial.

Source: NASA. Earth's atmosphere seems huge, but it's actually extremely thin – like a piece of paper wrapped around a soccer ball.

When did we discover the issue?

Early in the twentieth century, the prevailing notion was that people could alter climates locally (for instance, by cutting down forests and ploughing virgin fields) but not globally. Of course, the ice ages and other wrenching climate shifts of the past were topics of research. But few considered them an immediate threat, and hardly anyone thought humans could trigger worldwide climate change. A few pioneering thinkers saw the potential global impact of fossil-fuel use (see p. 27), but their views were typically dismissed by colleagues.

Starting in 1958, precise measurements of carbon dioxide confirmed its steady increase in the atmosphere. The first computer models of global climate in the 1960s, and more complex ones thereafter, supported the idea floated by mavericks earlier in the century: that the addition of greenhouse gases would indeed warm the climate. Finally, global temperature itself began to rise sharply in the 1980s, which helped raise the issue's profile among media and the public as well as scientists.

Couldn't the changes have natural causes?

The dramatic changes in climate we've seen in the past hundred years are not proof in themselves that humans are involved. As sceptics are fond of pointing out, Earth's atmosphere has gone through countless temperature swings in its 4.5 billion years. These are the results of everything from cataclysmic volcanic eruptions to changes in solar output and cyclic variations in Earth's orbit (see p. 196). The existence of climate upheavals in the past raises the question asked by naysayers as well as many people on the street: how can we be sure that the current warming isn't "natural" – ie caused by something other than burning fossil fuels?

That query has been tackled directly over the last decade or so by an increasing body of research, much of it through the Intergovernmental Panel on Climate Change (IPCC), a unique team that draws on the work of more than one thousand scientists. We'll refer often throughout this book to the IPCC's work (see p. 287 for more on the panel itself). Back in 1995, the IPCC's Second Assessment Report included a sentence that made news worldwide:

"The balance of evidence suggests a discernible human influence on global climate."

By 2001, when the IPCC issued its third major report, the picture had sharpened further:

"There is new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities."

And in its fourth report (2007), the IPCC spoke even more strongly:

"Human-induced warming of the climate system is widespread."

To support claims like these, scientists call on results from two critical types of work: detection and attribution studies. Detection research is meant to establish only that an unusual change in climate has occurred. Attribution studies try to find the likelihood that humans are involved.

One way to attribute climate change to greenhouse gases is by looking at the signature of that change and comparing it to what you'd expect from non-greenhouse causes. For example, over the past several decades, Earth's surface air temperature has warmed most strongly near the poles and at night. That pattern is consistent with the projections of computer models that incorporate rises in greenhouse gases. However, the pattern agrees less well with the warming that might be produced by other causes, including natural variations in Earth's temperature and solar activity.

As computer models have grown more complex, they've been able to incorporate more components of climate. This allows scientists to tease out the ways in which individual processes helped shape the course of the last century's warm-up. One such study, conducted at the US National Center for Atmospheric Research, examined five different factors: volcanoes, sulphate aerosol pollution, solar activity, greenhouse gases and ozone depletion. Each factor had a distinct influence. The eruption of Mount Pinatubo in 1991 helped cool global climate for several years. Sulphate pollution (see p. 190) peaked in the middle of the twentieth century, between World War II and the advent of environmentalism, and it may have helped produce the mid-century cool-down already discussed. Small ups and downs in solar output probably shaped the early-century warming and perhaps the mid-century cooling as well. However, the Sun can't account for the pronounced warming evident since the 1970s. The bottom line is that the model couldn't reproduce the most recent warming trend unless it included greenhouse gases.

Figure 1. The red and grey shaded areas show the departure in global temperature from the 1890–1919 average, as produced by four computer model simulations. The red and grey lines show averages of the four models. The grey runs factor in natural agents of climate change only; the red runs include both human and natural factors. The black line shows the temperature measured in the real world.

Couldn't some undiscovered phenomenon be to blame?

Although many people would love to find a "natural" phenomenon to blame for our warming planet, such as the relationship between clouds and cosmic rays (see p. 259), it's growing extremely unlikely that a suitable candidate will emerge. Even if it did, it would beg a difficult question: if some newly discovered factor can account for the climate change we've observed, then why aren't carbon dioxide and the other greenhouse gases producing the warming that basic physics tells us they should be?

And there's another catch. Any mystery process could just as easily be a cooling as a warming agent, and if it were to suddenly wane, it could leave us with even greater warming than we imagined possible. Trusting in the unknown, then, is a double-edged sword. As such, most scientists in the trenches trust in Occam's razor, the durable rule credited to the medieval English logician and friar William of Occam: "One should not increase, beyond what is necessary, the number of entities required to explain anything."

How do the rainforests fit into the picture?

The destruction of rainforests across the tropics is a significant contributor to climate change, accounting for roughly a fifth of recent human-produced CO2 emissions. Tropical forests hold nearly half of the carbon present in vegetation around the world. When they're burned to clear land, the trees, soils and undergrowth release CO2. Even if the land is eventually abandoned, allowing the forest to regrow, it would take decades for nature to reconcile the balance sheet through the growth of replacement trees that pull carbon dioxide out of the air. In addition to the CO2 from the fires, bacteria in the newly exposed soil may release more than twice the usual amount of another greenhouse gas, nitrous oxide, for at least two years. Brazil's National Institute for Amazon Research estimates that deforestation puts four times more carbon into the atmosphere than the nation's fossil-fuel burning does.

Rainforests also cool the climate on a more local level, their canopy helping to trap moisture and allow it to slowly evaporate, providing a natural air-conditioning effect. When the rainforest has been slashed and burned over large areas, hotter and dryer conditions often set in, although the exact strength of this relationship is difficult to quantify. Across eastern Brazil, where nearly 20% of the Amazonian rainforest has been destroyed, 2005 saw the region's worst drought in a century, perhaps related to changes in the nearby Atlantic and to rain-suppressing smoke from fires as well as to the deforestation itself.

By contrast, in mid-latitude and polar regions, forests actually tend to warm the climate (see p. 354).

Was Hurricane Katrina related to global warming?

It's impossible to tie any single weather event, including Katrina, to global warming. Several hurricanes of comparable strength have been observed across the Atlantic over the last century. The horrific damage in New Orleans was the result not only of Katrina's strength but also the storm's track, the weakness of levees and many other factors. That said, the waters of the Gulf of Mexico that fuelled Katrina were at near-record warmth. It appears that the tropics are part of a global trend towards ocean warming that goes hand in hand with atmospheric warming, and several studies have found an increase in hurricane intensity since the 1970s. See p. 131 for more on Katrina and other hurricanes and p.106 for details on oceanic changes.

Whatever happened to "global cooling"?

The planet did cool slightly from the 1940s to the 1970s, mainly in the Northern Hemisphere. However, despite a flurry of 1970s media reports on an imminent ice age (see p. 248), there was never anything approaching a scientific consensus on that issue. And while a slight decrease in the amount of sunlight reaching Earth – or "global dimming" – has been measured over the last few decades, it's not been enough to counteract the overall warming.

And the ozone hole?

There are a few links between ozone depletion and global warming, but for the most part they're two separate issues. The world community has already taken steps to address the Antarctic ozone hole, which is expected to disappear by the end of the twenty-first century. See p. 29.

The outlook

How hot will it get?

According to the 2007 IPCC report, the global average temperature is likely to rise anywhere from 1.1°C to 6.4°C (2.0.11.5°F) by 2080–2099, relative to 1980–1999. This range reflects uncertainty about the quantities of greenhouse gases we'll add to the atmosphere in coming decades and also about how the global system will respond to those gases. Some parts of the planet, such as higher latitudes, will heat up more than others. The warming will also lead to a host of other concerns – from intensified rains to melting ice – that are liable to cause more havoc than the temperature rise in itself.

Is global warming necessarily a bad thing?

Whether climate change is bad, good or neutral depends on your perspective. Some regions – and some species – may benefit, but many more will suffer intense problems and upheavals. And some of the potential impacts, such as a major sea-level rise, increased flooding and droughts, more major hurricanes and many species being consigned to extinction, are bad news from almost any perspective. So while it may be a bit of a reach to think in terms of "saving the planet" from global warming, it's perfectly valid to think about preserving a climate that's sustaining to as many of Earth's residents as possible.

Perhaps the more pertinent question is whether the people and institutions responsible for producing greenhouse gases will bear the impacts of their choices, or whether others will – including those who had no say in the matter. Indeed, people in the poorest parts of the world – such as Africa – will generally be least equipped to deal with climate change, even if the changes are no worse there than elsewhere. Yet those regions have released only a small fraction of the gases that are causing the changes.

Will anyone be killed or displaced?

Quantifying the human cost of climate change is exceedingly difficult. Weather-related disasters kill thousands of people each year, regardless of long-term changes in the climate. Many of the projected impacts of global warming on society are the combined effects of climate change and population growth (some claim the latter is far more important than the former). For this reason, it's hard to separate out how much of the potential human suffering is due to each factor.

In the decades to come, the warming of the planet and the resulting rise in sea level will likely begin to force people away from some coastlines. Low-lying islands are already vulnerable, and entire cities could eventually be at risk. The implications are especially sobering for countries such as Bangladesh, where millions of people live on land that may be inundated before the century is out.

Another concern is moisture – both too much and too little. In many areas rain appears to be falling in shorter but heavier deluges conducive to flooding. However, drought also seems to be becoming more prevalent. (See p. 59 for more on this seeming paradox.) Changes in the timing of rainfall and runoff could complicate efforts to ensure clean water for growing populations, especially in the developing world.

Warming temperatures may also facilitate the spread of vector-borne diseases such as malaria and dengue fever (see p. 156). The World Health Organization estimates that in 2000 alone, more than 150,000 people died as a result of direct and indirect climate-change impacts.

Will agriculture suffer?

That depends on where the farming and ranching is done. Global agricultural productivity is predicted to go up over the next century, thanks to the extra CO2 in the atmosphere and now-barren regions becoming warm enough to bear crops. However, the rich world looks set to reap the benefits: crop yields in the tropics, home to hundreds of millions of subsistence farmers, are likely to drop. See p. 162.

And wildlife?

Because climate is expected to change quite rapidly from an evolutionary point of view, we can expect major shocks to some ecosystems – especially in the Arctic – and possibly a wholesale loss of species. According to a 2004 study led by Chris Thomas of the University of Leeds and published in the journal Nature, climate change between now and 2050 may commit as many as 37% of all species to eventual extinction – a greater impact than that from global habitat loss due to human land use. Similar figures emerged from the 2007 IPCC report, which pegs the percentage of plant and animal species that are at risk from a temperature rise of 1.5–2.5°C (2.7–4.5°F) at 20–30%.

Will rising seas really put cities such as New York and London under water?

Not right away, but it may be only a matter of time. In its 2007 report, the IPCC projects that sea level will rise anywhere from 180 to 590mm (7–23") by 2090–2100. This range is smaller than in the IPCC's 2001 report, but it excludes some key uncertainties about how quickly warming will melt land-based ice. While the new IPCC figures don't signal a catastrophic sea-level rise this century, hurricanes and coastal storms on top of that rise could still cause major problems.

There's also the chance that sea-level rise over the next few decades and beyond could surprise us. The last few years have seen glaciers accelerating their seaward flow in many spots along the margins of Greenland and West Antarctica. Computer models don't depict the dynamics behind this speed-up very well, so it's not explicitly included in the IPCC projection, but the report does note the added risk at hand. If emissions continue to rise unabated through this century, the Greenland and/or West Antarctica ice sheets could be thrown into an unstoppable melting cycle that would raise sea level by more than 7m (23ft) each. This process would take some time to unfold – probably a few centuries, although nobody can pin it down at this point – but should it come to pass, many of the world's most beloved and historic cities would be hard-pressed to survive.

Will the Gulf Stream pack up, freezing the UK and northern Europe?

The Gulf Stream and North Atlantic Drift bring warm water (and with it warm air) from the tropical Atlantic to Northern Europe. This helps keep the UK several degrees warmer than it would otherwise be. Although this system is unlikely to pack up entirely, there is a possibility that it could be diminished by climate change. The reason is that increasing rainfall and snow-melt across the Arctic and nearby land areas could send more freshwater into the North Atlantic, pinching off part of the warm current. The best estimate is that the flow might weaken by 10.50% over the next century or so. That's probably not enough to offset global warming completely for the UK or northwest Europe, although it could certainly put a dent in it. In any case, the impacts would be much smaller – and would take much longer to play out – than the scenario dramatized in the film The Day After Tomorrow. See p. 275.

What can we do about it?

What's the Kyoto Protocol?

It's a United Nations-sponsored agreement among nations to reduce their greenhouse-gas emissions. Kyoto emerged from the UN Framework Convention on Climate Change, which was signed by nearly all nations at the 1992 mega-meeting popularly known as the Earth Summit. The framework pledges to stabilize greenhouse-gas concentrations "at a level that would prevent dangerous anthropogenic interference with the climate system". To put teeth into that pledge, a new treaty was needed, one with binding targets for greenhouse-gas reductions. That treaty was finalized in Kyoto, Japan, in 1997 after years of negotiations.

From the start, the chances that the Kyoto Protocol would become international law were tenuous. The US and Australia indicated early on that they wouldn't ratify it, citing the absence of binding targets for developing countries. But the protocol itself required ratification by enough industrialized countries to represent 55% of the developed world's CO2 output. With the US and Australia out of the picture, virtually every other first-world country would have to ratify the treaty, a process that took seven uncertain years. Finally, Russia's decisive vote in late 2004 brought Kyoto into force the following year. As of mid-2007, 172 states had ratified the treaty (see map).

Under Kyoto, industrialized nations have pledged to cut their yearly emissions of carbon, as measured in six greenhouse gases, by varying amounts, averaging 5.2%, by 2012 as compared to 1990. That equates to a 29% cut in the values that would have otherwise occurred. However, the protocol didn't become international law until more than halfway through the 1990–2012 period. By that point, emission amounts had risen substantially in many countries: over 20% in Canada, for instance. And in some countries exempt from the Kyoto rules, particularly China, emission levels are skyrocketing.

Will Kyoto make a difference?

It appears that few if any of the world's big economies will meet their Kyoto targets by 2012. Even if they did, it would only make a tiny dent in the world's ever-increasing output of greenhouse gases. Reducing greenhouse gas emissions by a few percent over time is akin to overspending your household budget by a decreasing amount each year: your debt still piles up, if only at a slower pace.

The century-long lifespan of atmospheric CO2 means that the planet is already committed to a substantial amount of greenhouse warming. Even if we turned off every fuel-burning machine on Earth tomorrow, climate modellers tell us that the world would warm at least another 0.5°C (0.9°F) as the climate adjusts to greenhouse gases we've already emitted. The bottom line is that we won't come close to keeping greenhouse heating in check until changes in technology and lifestyle enable us to pull back far beyond our current emission levels, or unless we find some safe method to remove enormous amounts of carbon from the atmosphere, or both. That's a tall order – but if we're determined to reduce the risk of a wide range of climate impacts, we have no choice but to fulfil it.

Will we reach a "tipping point"?

The effects of climate change aren't expected to be strictly linear. A 4°C warming could be more than twice as risky as a 2°C warming, because of positive-feedback processes that tend to amplify change and make it worse. The challenge is to identify the points at which the most dangerous positive feedbacks will kick in. For instance, scientists consider it likely that the Greenland ice sheet will begin melting uncontrollably if global temperatures climb much more than 2°C (3.6°F). Because of the implications for coastal areas, as noted above, this is a particularly worrisome threshold.

Since each positive feedback has its own triggering mechanism, there is no single temperature agreed upon as a tipping point for Earth as a whole. However, scientists, governments and activists have worked to identify useful targets. One goal adopted by the European Union, as well as many environmental groups, is to limit global temperature rise to 2°C (3.6°F) over pre-industrial levels. But that ceiling looks increasingly unrealistic – we're already close to 40% of the way there, and only the lower fringes of the latest IPCC projections keep us below the 2°C threshold by century's end.

Another approach is to set a stabilization level of greenhouse gases – a maximum concentration to be allowed in the atmosphere – such as 500 parts per million, as compared to 270–280ppm in pre-industrial times and about 380ppm today. You can learn more about these and other goals in The Predicament, p.278.

Which countries are emitting the most greenhouse gases?

For many years the United States was in first place, with 30% of all of the human-produced greenhouse emissions to date and about 20% of the current yearly totals – despite having only a 5% share of global population. However, China is now taking the lead. Its emissions are much lower per capita, but due to its growing population and affluence, China will overtake the US as the world's leading greenhouse emitter by 2008. As shown in the diagram on p.294, the world's industrialized countries varied widely in how much they have increased or decreased their total emissions since 1990. Some of the decreases were due to efficiency gains, while others were due to struggling economies.

Does the growth of China and India make a solution impossible?

Not necessarily. Although its growth in its coal production is hugely worrisome, China is already making progress on vehicle fuel efficiency and other key standards. And because so much of the development in China and India is yet to come, there's a window of opportunity for those nations to adopt the most efficient technologies possible. At the same time, the sheer numbers in population and economic growth for these two countries are daunting indeed . all the more reason for prompt international collaboration on technology sharing and post-Kyoto diplomacy.

If oil runs out, does that solve the problem?

Hardly. It's true that if oil resources do "peak" in the next few years, as some experts believe, we're likely to see economic downswings, and those could reduce oil-related emissions, at first over periods of a few years and eventually for good. The same applies to natural gas, although that peak could arrive decades later. Then, the question becomes what fuel sources the world will turn to: coal, nuclear, renewables or some combination of the three. If the big winner is coal – or some other, less-proven fossil source such as shale or methane hydrates – it raises the potential for global warming far beyond anything in current projections.

Even if renewables win the day later in this century, we're still left with the emissions from today's stocks of oil, gas and coal, many of which would likely get burned between now and that eco-friendly transition. With this in mind, research has intensified on sequestration – how carbon might be safely stored underground. The idea appears promising, but big questions remain. See p. 310.

Won't nature take care of global warming in the long run?

Only in the very long run. The human enhancements to the greenhouse effect could last the better part of this millennium. Assuming that it takes a century or more for humanity to burn through whatever fossil fuels it's destined to emit, it will take hundreds more years for those greenhouse gases to be absorbed by Earth's oceans.

There are few analogies in the geological past for such a drastic change in global climate over such a short period, so it's impossible to know what will happen after the human-induced greenhouse effect wanes. All else being equal, cyclical changes in Earth's orbit around the Sun can be expected to trigger an ice age sometime within the next 50,000 years, and other warmings and coolings are sure to follow, as discussed in the box on p. 224. In the meantime, we'll have our hands full dealing with the next century and the serious climate changes that our way of life may help bring about.

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