Polar bear near arcticiStockphoto

There is no real debate about whether climate change is occurring. The only dissent comes from the fringes, and generally from those whose research institutions or blogs are devoted, for ideological or other reasons, to attempting to debunk the notion that human activities are altering the planet's climate. But for many, the discussion, such as it is, can seem confusing. Is the Arctic Ocean predicted to be ice-free by the summer of 2100, or 2050, or 2030? And what exactly does ice-free mean? Are hurricanes supposed to become more frequent, or less frequent but more intense?

For scientists studying the impacts of climate change, such questions - and answers - are constantly being revised and refined as more information is gathered, models are fine-tuned, and feedbacks are better understood. But even as they focus their forecasts, those scientists are increasingly seeing the evidence of global warming happening right now, many of them in line with predictions and some of them even more severe and more rapid than anticipated. The following list provides a sampling of some of the key pieces of evidence that climate change is not just a prediction, it is already underway.

1. Carbon Dioxide Concentrations in the Atmosphere Are Increasing

This is the first, key point. By analyzing air bubbles trapped in the ice of Antarctica and Greenland, scientists have been able to determine that over the past 650,000 or so years, atmospheric concentrations of carbon dioxide (CO2) varied between 180 and 300 parts per million (ppm), and in the years immediately prior to the Industrial Revolution in the mid-eighteenth century stood at approximately 280 ppm. Since then, however, that figure has steadily increased; by the time continuous monitoring began at Mauna Loa Observatory in Hawaii, it had climbed above 310 and is now closing in on 400.

Because we know that carbon dioxide is a greenhouse gas, we can reasonably infer that increasing the amount of CO2 that enters the atmosphere, particularly at the level of 90 million tons a day, will increase the greenhouse properties of the atmosphere and thus lead to warming. (And while, as skeptics often like to point out, water vapor is a more powerful greenhouse gas than CO2, we are not emitting vast amounts of water vapor every day; indeed, the atmosphere can only hold a certain amount of atmosphere at a time. But, by warming the atmosphere, increased CO2 levels enable the retention of greater amounts of water vapor, thus enhancing warming.

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Furthermore, scientists know, from analyzing the isotopes of the carbon in the atmosphere, that the increase in atmospheric CO2 is the result of burning fossil fuels and forests, and not the result of natural processes. Accordingly, a National Research Council study was able to point out back in 2001 that, "Greenhouse gases are accumulating in Earth's atmosphere as a result of human activities, causing surface air temperatures and sub-surface ocean temperatures to rise." Because, indeed, as that study noted, "Temperatures are, in fact, rising."

Holidaymakers crowd a beach resort during a summer scorcher in Dalian city, northeast China's Liaoning province, in July 2012.Corbis

Whether measured from land or from satellite, it is clear that global temperatures are increasing.

Averaged over all land and ocean surfaces, global mean temperatures have increased by approximately 1.33 degrees Fahrenheit (0.74 degrees Celsius) over the past century. More than half of this warming—about 0.72 °F (0.4 °C)—has occurred since 1979. Because oceans tend to warm and cool more slowly than land areas, continents have warmed the most (about 1.26 °F or 0.7 °C since 1979), especially over the Northern Hemisphere.

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Furthermore, the rate of increase is, well, increasing.

Even with year-to-year natural variations, underlying global surface and lower atmosphere warming trends are maintaining an upward trajectory.

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Hikers approach the terminus of Athabasca Glacier. Athabasca Glacier occupied most of the valley in the 1960s, but in the last fifty years global warming has melted most of it. It may be gone by 2030.Ben Cooper/Corbis

Of course, Earth's climate has historically undergone numerous significant shifts. It has been, at various times, both much warmer and much colder on average than it is now. How do we know that what is happening now is not one of those natural cycles? Well, for one thing, none of the natural forces - tilts in the planet's axis, wobbles in its orbit, or increased solar activity - are factors (the Sun, in fact, has been going through a slight cooling cycle even as temperatures on Earth have increased). Another clue that this is human-caused is the speed at which the change is occurring.

A graphic representation of the rate of recent change has long been provided by the famous (or, depending on your point of view, infamous) "hockey stick" reconstruction, which shows a period of relatively stable temperatures followed by an upward surge beginning in the latter half of the twentieth century. Numerous studies have reinforced that finding. In 2008, 14 years after the initial reconstruction, a new version extended the stick's 'handle' back over a millennium, showing that "recent warmth appears anomalous for at least the past 1,300 years."

This year, a brand new study combined data from 73 sites around the world to show that temperatures today are warmer than they have been during 70-80 percent of the Holocene Epoch - the stretch of time, beginning about 11,300 years ago, since the last major Ice Age - and that, under all existing scenarios for different levels of greenhouse gas emissions, virtually every model shows temperatures will exceed the very hottest periods during that time. According to Candace Major of the National Science Foundation, "This research shows that we've experienced almost the same range of temperature change since the beginning of the industrial revolution as over the previous 11,000 years of Earth history -- but this change happened a lot more quickly."

Sea kayakers in Spitzberg, Svalbard Islands, Norway. Frilet Patrick/Hemis/Corbis

The extent of summer sea ice covering the Arctic Ocean is now decreasing by a rate of about 13 percent per decade, compared to the 1979-2000 average. The six lowest annual minimum extents on record have been in the last six years; in 2012, Arctic sea ice extent reached its lowest level in the satellite record, fully 760,000 square kilometers (293,000 square miles) below the previous record, which occurred in 2007. That difference is an area about the size of the state of Texas. The 2012 minimum was in turn 3.29 million square kilometers (1.27 million square miles) (or 49 percent) below the 1979 to 2000 average minimum, representing an area nearly twice the size of the state of Alaska.

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Much of the ice that remains is thinner, first-year ice, rather than the thicker multiyear ice that has characterized the Arctic Ocean in the past. Because it forms each winter, it is more vulnerable to break-up and melt the following summer. The growing dominance of this thinner ice means that the volume of Arctic Ocean sea ice has also collapsed, from approximately 16,855 cubic kilometers in 1979 to roughly 3,261 cubic kilometers in 2012. In other words, Arctic sea ice has lost 80 percent of its volume. As Arctic ice becomes smaller in extent and thinner in volume, it becomes increasingly vulnerable to further melt, prompting National Snow and Ice Data Center (NSIDC) Director Mark Serreze to say it is in a "death spiral." Some experts think we could see nearly ice-free summers in the Arctic Ocean in a decade if present trends continue.

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This is a satellite image of a massive iceberg calving from the Petermann Glacier in Greenland, collected on July 19, 2012.DigitalGlobe via Getty Images

Evidence is mounting that Greenland - the second-largest ice sheet in the world after Antarctica - is losing mass at an accelerating rate. Much of this loss is occurring along Greenland's edges, where rapidly-moving glaciers and ice streams are discharging more ice into the ocean than is being accumulated on the ice cap. To date, much of this ice loss has occurred in the southern part of the landmass, but it now appears that these losses are spreading to the northwest. Additionally, in July 2012, Greenland saw melting occur across approximately 97 percent of its surface ice.

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How much ice is Greenland losing? At present, almost 300 gigatonnes per year - which, on the face of it, means we'd have to wait a long time (about 10,500 years) until the ice sheet completely dissipated. But of course, the ice sheet will contribute significantly to sea level rise long before it disappears entirely. The rate of ice loss is increasing so rapidly that just ten years ago it was extrapolated that total ice sheet dissipation would happen in 22,000 years. In other words, the amount of time until the Greenland ice sheet vanishes has been halved - reduced by 10,000 years - in just a decade. And the rate of ice loss is still increasing. Even when the news is good, it's bad: a recent study that found the Greenland ice sheet may be more stable than we thought concluded that this may mean the Antarctic ice sheet is less stable than previously believed.

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Gentoo Penguins (Pygoscelis papua) walk along rocky shoreline past icicles hanging from tidal ice shelf in Cuverville Island, Antarctica.Paul Souders/Corbis

In 1995, the Larsen-A ice shelf on the Antarctic Peninsula collapsed. Seven years later, the adjoining Larsen B ice shelf followed suit. The same year that Larsen A disappeared, the Prince Gustav ice shelf, 60 kilometers to the north, did the same. The Wordie Ice Shelf broke away from the Antarctic Peninsula and vanished in 2009. The Wilkins Ice Shelf has been splintering for several years and is now 'hanging by a thread' to the coast.

Much of Antarctica is warming, but the Antarctic Peninsula - the region that reaches northward toward the tip of South America - is actually the most rapidly-warming part of the Southern Hemisphere, having seen temperatures increase by about 2.8 degrees Celsius over the last 50 years. Even when ice shelves don't collapse, their surfaces partially melt during summer - and according to new research, they are doing so now at a rate more than ten times greater than 600 years ago. According to one of the study's authors, "the melting we observe at this site in the past few decades has no similar analogue in the past 1000 years - here we can say the level of melting observed today is unique in the context of the past 1000 years."

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Corals are seen at the Great Barrier Reef. Rising carbon dioxide levels in the world's oceans due to climate change, combined with rising sea temperatures, could accelerate coral bleaching, destroying some reefs before 2050.Handout/Reuters/Corbis

Inevitably, much of the climate attention focuses on the planet's surface - because that's the part where we live. But 90 percent of global warming goes into heating, not the land or atmosphere, but the ocean. Because it takes far more energy to heat up the entire ocean than the lower atmosphere or a surface layer of ice, the amount that the ocean has warmed is much less than on land: on average, about 0.025 degrees Celsius a decade - or slightly more than one-tenth of a degree Celsius over the last 50 years. Interestingly, this warming is not just affecting the surface of the sea; 30 percent of ocean warming has been taking place in waters deeper than 700 meters, and some has even occurred in the deepest, abyssal waters of the ocean. This deep-water warming is most pronounced in the Southern Ocean around Antarctica, which is warming at roughly 0.03 degrees Celsius a decade, although most abyssal oceans are warming at around one-tenth that rate.

Sea level rise is swamping coasts; Rodanthe in the Outer Banks of North Carolina is pictured.Andrew Kemp, Yale University

Despite skeptic assertions to the contrary, multiple measurements - using both satellites and tide gauges - show a rise in global sea levels. On average, since 1993, the sea has been rising by 3.18 mm per year, primarily as a consequence of thermal expansion due to warming, and to the melting of ice sheets.

However, this rise contains marked spatial and temporal variations. Regionally and locally, changes may be greater or lower, affected not only by thermal expansion but factors ranging from local wind patterns to the mining of groundwater aquifers. In late 2010 and early 2011, sea levels underwent a sharp fall, a fact that was gleefully leaped upon by skeptics; but NASA researchers pointed out that 2010 saw a transition from a strong El Niño to “one of the strongest La Niñas in recent memory."

This sudden shift in the Pacific “changed rainfall patterns all across the globe, bringing massive floods to places like Australia and the Amazon basin." The water to power that rainfall came from the ocean, the level of which consequently dipped. Since then, sea level has resumed rising at an accelerated clip of approximately 10 mm a year. Researchers believe that, even as the overall trend will remain clearly upward, it may increasingly manifest in rapid short-term divergences - deeper potholes, such as that of 2010-11, and steeper speed bumps, such as the one we are witnessing now.

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Joshua Trees all over Southern California bloomed in April 2013 for the first time simultaneously, and for the first time in over three years due to drought conditions. Botanists theorize that the massive blooms may be due to global warming.Ted Soqui Photography/Corbis

The fact that the ocean is warming - and particularly the discovery of warming in the deep ocean - underlines an important point: the planet is accumulating more heat. Recently, skeptics have argued that an apparent reduction in increases in surface temperature somehow suggests that climate change is "slowing down" or even non-existent. But the heat trapped by greenhouse gases isn't just absorbed and radiated by land, and doesn't just heat the atmosphere. Satellite measurements of incoming and outgoing radiation, as well as studies that have combined measurements for land, ice, atmosphere and the ocean (such as this one, this one, and this one) have all shown one unmistakable fact: the planet is accumulating heat, and doing so at a growing rate. When you consider all the heat building up in the entirety of our climate, global warming has actually accelerated over the past 16 years that deniers claim nothing is happening. So the heat build-up continues unabated. Not only is global warming not slowing down, it is increasing. If and when that "hidden heat" returns to the atmosphere, the impact will likely be felt strongly.

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Residents walk past damaged houses in in Breezy Point, a neighborhood in the Queens borough of New York which was left devastated by Hurricane Sandy November 12, 2012. Picture taken November 12, 2012.Adrees Latif/Corbis

Making a direct connection between climate change and extreme weather events is not straightforward. Weather, after all, is short-term and highly variable. There have always been and always will be storms and heatwaves. Climate scientists are careful not to scribe any specific weather event to global warming. However, climate creates the conditions in which weather takes place - as one expert has explained it, "climate trains the boxer, but weather throws the punches" - and scientists have long suspected that a changing climate will make certain weather events more likely and others more extreme. Many researchers say we are increasingly seeing those predicted linkages show themselves.

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For example, a warming ocean, while actually making it more difficult for hurricanes to form, is leading to the hurricanes that do form to become stronger. When major storms do strike, higher sea levels will result in greater storm surges and coastal flooding. As the Arctic warms, circumpolar wind patterns are becoming disrupted, altering the course of the jet stream, which steers weather systems from west to east around the northern hemisphere. As a consequence, says a recent study, the jet stream is becoming “wavier," with steeper troughs and higher ridges. Weather systems in turn are progressing more slowly, raising the chances for long-duration extreme events, like droughts, floods, extreme snowfall in winter, and heat waves. Recent studies have attributed some recent rainfall extremes to climate change, with a warmer atmosphere able to hold more moisture, while others indicate that many recent heat-waves would not have occurred without global warming.

Weather events will always be subject to natural variability. But weather extremes are one predicted consequence of a changing climate, and the evidence is growing that recent examples of those extremes are not isolated, but rather harbingers of a new normal in a warming world.