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GREENHOUSE GASES such as carbon dioxide (CO2) absorb heat (infrared radiation) emitted from Earth’s surface. Increases in the atmospheric concentrations of these gases cause Earth to warm by trapping more of this heat. Human activities—especially the burning of fossil fuels since the start of the Industrial Revolution—have increased atmospheric CO2 concentrations by about 40%, with more than half the increase occurring since 1970. Since 1900, the global average surface temperature has increased by about 0.8 °C (1.4 °F). This has been accompanied by warming of the ocean, a rise in sea level, a strong decline in Arctic sea ice, and many other associated climate effects. Much of this warming has occurred in the last four decades.

Detailed analyses have shown that the warming during this period is mainly a result of the increased concentrations of CO2 and other greenhouse gases. Continued emissions of these gases will cause further climate change, including substantial increases in global average surface temperature and important changes in regional climate. The magnitude and timing of these changes will depend on many factors, and slowdowns and accelerations in warming lasting a decade or more will continue to occur. However, long-term climate change over many decades will depend mainly on the total amount of CO2 and other greenhouse gases emitted as a result of human activities.

Since the mid-1800s, scientists have known that CO2 is one of the main greenhouse gases of importance to Earth’s energy balance. Direct measurements of CO2 in the atmosphere and in air trapped in ice show that atmospheric CO2 increased by about 40% from 1800 to 2012. Measurements of different forms of carbon (isotopes, see Question 3) reveal that this increase is due to human activities. Other greenhouse gases (notably methane and nitrous oxide) are also increasing as a consequence of human activities. The observed global surface temperature rise since 1900 is consistent with detailed calculations of the impacts of the observed increase in atmospheric CO2 (and other human-induced changes) on Earth’s energy balance.

Different influences on climate have different signatures in climate records. These unique fingerprints are easier to see by probing beyond a single number (such as the average temperature of Earth’s surface), and looking instead at the geographical and seasonal patterns of climate change. The observed patterns of surface warming, temperature changes through the atmosphere, increases in ocean heat content, increases in atmospheric moisture, sea level rise, and increased melting of land and sea ice also match the patterns scientists expect to see due to rising levels of CO2 and other human-induced changes (see Question 5).

The expected changes in climate are based on our understanding of how greenhouse gases trap heat. Both this fundamental understanding of the physics of greenhouse gases and fingerprint studies show that natural causes alone are inadequate to explain the recent observed changes in climate. Natural causes include variations in the Sun’s output and in Earth’s orbit around the Sun, volcanic eruptions, and internal fluctuations in the climate system (such as El Niño and La Niña). Calculations using climate models (see infobox, p.20) have been used to simulate what would have happened to global temperatures if only natural factors were influencing the climate system. These simulations yield little warming, or even a slight cooling, over the 20th century. Only when models include human influences on the composition of the atmosphere are the resulting temperature changes consistent with observed changes.

In nature, CO2 is exchanged continually between the atmosphere, plants and animals through photosynthesis, respiration, and decomposition, and between the atmosphere and ocean through gas exchange. A very small amount of CO2 (roughly 1% of the emission rate from fossil fuel combustion) is also emitted in volcanic eruptions. This is balanced by an equivalent amount that is removed by chemical weathering of rocks. The CO2 level in 2012 was about 40% higher than it was in the nineteenth century. Most of this CO2 increase has taken place since 1970, about the time when global energy consumption accelerated. Measured decreases in the fraction of other forms of carbon (the isotopes 14C and 13C) and a small decrease in atmospheric oxygen concentration (observations of which have been available since 1990) show that the rise in CO2 is largely from combustion of fossil fuels (which have low 13C fractions and no 14C).

Deforestation and other land use changes have also released carbon from the biosphere (living world) where it normally resides for decades to centuries. The additional CO2 from fossil fuel burning and deforestation has disturbed the balance of the carbon cycle, because the natural processes that could restore the balance are too slow compared to the rates at which human activities are adding CO2 to the atmosphere. As a result, a substantial fraction of the CO2 emitted from human activities accumulates in the atmosphere, where some of it will remain not just for decades or centuries, but for thousands of years. Comparison with the CO2 levels measured in air extracted from ice cores indicates that the current concentrations are higher than they have been in at least 800,000 years (see Question 6).

The observed warming in the lower atmosphere and cooling in the upper atmosphere provide us with key insights into the underlying causes of climate change and reveal that natural factors alone cannot explain the observed changes. In the early 1960s, results from mathematical/physical models of the climate system first showed that human-induced increases in CO2 would be expected to lead to gradual warming of the lower atmosphere (the troposphere) and cooling of higher levels of the atmosphere (the stratosphere). In contrast, increases in the Sun’s output would warm both the troposphere and the full vertical extent of the stratosphere.

At that time, there was insufficient observational data to test this prediction, but temperature measurements from weather balloons and satellites have since confirmed these early forecasts. It is now known that the observed pattern of tropospheric warming and stratospheric cooling over the past 30 to 40 years is broadly consistent with computer model simulations that include increases in CO2 and decreases in stratospheric ozone, each caused by human activities. The observed pattern is not consistent with purely natural changes in the Sun’s energy output, volcanic activity, or natural climate variations such as El Niño and La Niña. Despite this agreement between the global-scale patterns of modelled and observed atmospheric temperature change, there are still some differences. The most noticeable differences are in the tropical troposphere, where models currently show more warming than has been observed, and in the Arctic, where the observed warming of the troposphere is greater than in most models.

All major climate changes, including natural ones, are disruptive. Past climate changes led to extinction of many species, population migrations, and pronounced changes in the land surface and ocean circulation. The speed of the current climate change is faster than most of the past events, making it more difficult for human societies and the natural world to adapt. The largest global-scale climate variations in Earth’s recent geological past are the ice age cycles (see infobox, p.B4), which are cold glacial periods followed by shorter warm periods [Figure 3]. The last few of these natural cycles have recurred roughly every 100,000 years. They are mainly paced by slow changes in Earth’s orbit which alter the way the Sun’s energy is distributed with latitude and by season on Earth.

These changes alone are not sufficient to cause the observed magnitude of change in temperature, nor to act on the whole Earth. Instead they lead to changes in the extent of ice sheets and in the abundance of CO2 and other greenhouse gases which amplify the initial temperature change and complete the global transition from warm to cold or vice versa. Recent estimates of the increase in global average temperature since the end of the last ice age are 4 to 5 °C (7 to 9 °F). That change occurred over a period of about 7,000 years, starting 18,000 years ago. CO2 has risen by 40% in just the past 200 years, contributing to human alteration of the planet’s energy budget that has so far warmed Earth by about 0.8 °C (1.4 °F). If the rise in CO2 continues unchecked, warming of the same magnitude as the increase out of the ice age can be expected by the end of this century or soon after. This speed of warming is more than ten times that at the end of an ice age, the fastest known natural sustained change on a global scale.

The present level of atmospheric CO2 concentration is almost certainly unprecedented in the past million years, during which time modern humans evolved and societies developed. The atmospheric CO2 concentration was however higher in Earth’s more distant past (many millions of years ago), at which time palaeoclimatic and geological data indicate that temperatures and sea levels were also higher than they are today. Measurements of air in ice cores show that for the past 800,000 years up until the 20th century, the atmospheric CO2 concentration stayed within the range 170 to 300 parts per million (ppm), making the recent rapid rise to nearly 400 ppm over 200 years particularly remarkable [figure 3]. During the glacial cycles of the past 800,000 years both CO2 and methane have acted as important amplifiers of the climate changes triggered by variations in Earth’s orbit around the Sun.

As Earth warmed from the last ice age, temperature 7 continued 10 Climate Change n Q&A Is there a point at which adding more CO2 will not cause further warming? No. Adding more CO2 to the atmosphere will cause surface temperatures to continue to increase. As the atmospheric concentrations of CO2 increase, the addition of extra CO2 becomes progressively less effective at trapping Earth’s energy, but surface temperature will still rise. Our understanding of the physics by which CO2 affects Earth’s energy balance is confirmed by laboratory measurements, as well as by detailed satellite and surface observations of the emission and absorption of infrared energy by the atmosphere. Greenhouse gases absorb some of the infrared energy that Earth emits in so-called bands of stronger absorption that occur at certain wavelengths. Different gases absorb energy at different wavelengths. CO2 has its strongest heat-trapping band centred at a wavelength of 15 micrometres (millionths of a metre), with wings that spread out a few micrometres on either side.

There are also many weaker absorption bands. As CO2 concentrations increase, the absorption at the centre of the strong band is already so intense that it plays little role in causing additional warming. However, more energy is absorbed in the weaker bands and in the wings of the strong band, causing the surface and lower atmosphere to warm further. and CO2 started to rise at approximately the same time and continued to rise in tandem from about 18,000 to 11,000 years ago. Changes in ocean temperature, circulation, chemistry and biology caused CO2 to be released to the atmosphere, which combined with other feedbacks to push Earth into an even warmer state. For earlier geological times, CO2 concentrations and temperatures have been inferred from less direct methods. Those suggest that the concentration of CO2 last approached 400 ppm about 3 to 5 million years ago, a period when global average surface temperature is estimated to have been about 2 to 3.5°C higher than in the pre-industrial period. At 50 million years ago, CO2 may have reached 1000 ppm, and global average temperature was probably about 10°C warmer than today. Under those conditions, Earth had little ice, and sea level was at least 60 metres higher than current levels.

During El Niño events, global average temperatures tend to be warmer than normal, while during La Niña events, temperatures tend to be cooler than normal. Since 1950, both the cooler-than-normal La Niña periods and the warmer-than-normal El Niño periods have been warming over time. This shows that this natural cycle appears to be superimposed on a longer-term warming trend.

Earth’s lower atmosphere is becoming warmer and moister as a result of human-emitted greenhouse gases. This gives the potential for more energy for storms and certain severe weather events. Consistent with theoretical expectations, heavy rainfall and snowfall events (which increase the risk of flooding) and heatwaves are generally becoming more frequent. Trends in extreme rainfall vary from region to region: the most pronounced changes are evident in North America and parts of Europe, especially in winter. Attributing extreme weather events to climate change is challenging because these events are by definition rare and therefore hard to evaluate reliably, and are affected by patterns of natural climate variability.

For instance, the biggest cause of droughts and floods around the world is the shifting of climate patterns between El Niño and La Niña events. On land, El Niño events favour drought in many tropical and subtropical areas, while La Niña events promote wetter conditions in many places, as has happened in recent years. These short-term and regional variations are expected to become more extreme in a warming climate. There is considerable uncertainty about how hurricanes are changing because of the large natural variability and the incomplete observational record. The impact of climate change on hurricane frequency remains a subject of ongoing studies. While changes in hurricane frequency remain uncertain, basic physical understanding and model results suggest that the strongest hurricanes (when they occur) are likely to become more intense and possibly larger in a warmer, moister atmosphere over the oceans. This is supported by available observational evidence in the North Atlantic. Some conditions favourable for strong thunderstorms that spawn tornadoes are expected to increase with warming, but uncertainty exists in other factors that affect tornado formation, such as changes in the vertical and horizontal variations of winds.

Land and submarine volcanoes emit about 260 million metric tons of CO2 per year, researchers estimate. Human use of fossil fuels and CO2 emissions through land-use changes deliver more than

30 billion metric tons of the gas a year to the atmosphere – more than 100 times the annual emissions from volcanoes.

Without greenhouse gases the temperature on Earth would average 0 degrees F. With the greenhouse effect, the average temperature on Earth is 59 degrees F.

Electricity accounted for 32 percent of total US greenhouse gas emissions in 2012. The majority of that came from burning coal. Transportation is the second largest source of greenhouse gas emissions in the US. [Editor’s note: This response has been updated for clarification. Electricity accounted for 32 percent of total US greenhouse gas emissions in 2012. For just carbon dioxide emissions, the sector accounted for 38 percent of US emissions. In either case, it was the largest source of emissions in 2012.]

The average global sea level has risen 6.7 inches, or 17 centimeters, in the past century. However, the rate of change has doubled in the last decade. Between 1870 and 2000, the sea level rose at an average of 1.70 millimeters a year. From 1993-present it has risen at an average of 3.17 millimeters a year. This is caused by thermal expansion caused by warmer ocean water and the melting of glaciers and the polar ice caps. Sea-level changes vary widely by location. The East coast of the US is a hot spot for sea-level changes, which could impact major US cities.

In June 2014, scientists recorded the largest Antarctic sea ice extent ever. The sea ice covered about 15.26 million square kilometers. This surpassed the previous record, set in 2010, by 260,000 square kilometers. Some scientists see evidence that the air around Antarctica is warming, and warmer air can hold more moisture. That moisture leads to more snow falling on the ocean around Antarctica, which makes the ocean less salty and dense. Less salty water is able to freeze at a higher temperature.

1978 was the first time satellites were used to observe ice at the Arctic. Since then, the yearly minimum sea ice extent during the summer melt season has decreased by 40 percent. This has increased the amount of open water (lower albedo than ice) available to absorb sunlight and return it to the atmosphere later as heat. This change in albedo has has amplified the warming in the Arctic. [Editor’s note: The question has been updated to specify “annual minimum” of Arctic sea ice.]

1998 is the only year from the 20th century to be one of the 10 hottest years ever recorded. The other nine are years from the 21st century. Here are the top 10 hottest years on record, starting with the hottest: 2010, 2005, 1998, 2013, 2003, 2002, 2006, 2009, 2007, 2004, 2012.

In January 2015, NASA and NOAA announced that 2014 was the hottest on record, although scientists added that the margin of error is such that 2014, 2010, and 2005 are effectively statistically tied for first place

Ocean acidification: Oceans have absorbed about a third of the carbon dioxide created from human activities, decreasing the pH level. The ocean holds about 50 times more carbon than the atmosphere. Due to acidification, less calcium carbonate is available for building structures such as shells or coral reefs. Other effects include lower growth rates, reduce the immune responses among some species.

20 tons of carbon dioxide is the amt of ave American emittance annualy: When national emissions are calculated on a per-person basis, the average American consumes goods and services that generate about 19.74 tons of CO2 per year, according to the United Nations Statistics Division. Qatar tops the list of per capita emitters, with the average Qatari spewing more than 55 tons of CO2 into the atmosphere. On a national basis, China is the top CO2 emitter, followed by the US, India, Russia, and Japan, based on 2012 emissions estimates.

The ozone hole refers to destruction of ozone molecules in the stratosphere, mainly concentrated over Antarctica. A smaller, less dramatic hole appear over the Arctic. Ozone has thinned due mainly to fluorine-, chlorine-, and bromine-based industrial compounds. These were incorporated into widely used products like refrigerants, propellants for spray cans, and foam furniture cushions. The decline in ozone over Antarctic appears to have been arrested in the early to mid 1990s, and its size has been trending smaller since the mid 2000s.

The ozone hole refers to destruction of ozone molecules in the stratosphere, mainly concentrated over Antarctica. A smaller, less dramatic hole appear over the Arctic. Ozone has thinned due mainly to fluorine-, chlorine-, and bromine-based industrial compounds. These were incorporated into widely used products like refrigerants, propellants for spray cans, and foam furniture cushions. The decline in ozone over Antarctic appears to have been arrested in the early to mid 1990s, and its size has been trending smaller since the mid 2000s.

400 ppm: This is the highest concentration of CO2 the atmosphere has held in at least 800,000 years and more likely in 20 million years. Scientists expect the atmosphere to reach that average level on an annual basis next year. CO2 emissions are exceeding levels that would have provided a 50-50 chance of holding the increase in global average temperatures to about 2 degrees Celsius by 2100.

Over the past 30 to 40 years, as the troposphere (the major layer of the atmosphere closest to the surface), resumed warming after a 30- to 40-year pause, the stratosphere has cooled. The main reason is that fluorine-, chlorine-, and bromine-based industrial compounds have destroyed stratospheric ozone, which is a greenhouse gas. But the stratosphere also has been cooling because greenhouse gases in the troposphere have trapped more heat there.

Since 1900, the temperature of the atmosphere has risen 1.4 degrees F. Each of the last three decades has been warmer than any other decade.

From 1800 to 2012, the amount of atmospheric CO2 has increased

: Since pre-industrial times, the amount of carbon dioxide in the atmosphere has increased by 40 percent. Half of that increase has come since 1970. Additionally, the amount of methane in the atmosphere has increased by 150 percent.

Plants turn carbon dioxide into sugars and starches during photosynthesis, locking the carbon up in their tissues. On average, during their lifetimes plants return to the atmosphere about half of the CO2 they absorb. When they die and decompose, virtually all of their carbon returns to the atmosphere as CO2. If plants (or animals) are buried before they can decompose on the surface and remain buried, heat and pressure over millions of years turn them into deposits of coal, oil, and gas. CO2 from coal burned in a power plant today represents a return to the atmosphere of carbon long sequestered deep underground, away from the climate system.

The Earth’s surface absorbs the sun’s visible light and emits infrared radiation back toward space. Greenhouse gases absorb and re-emit the infrared radiation in all directions, including back toward Earth.

About 30 percent of the sun’s visible light is reflected by clouds, atmospheric particles, snow, and ice. That leaves 70 percent to be absorbed by the oceans, land, and plant cover to be returned to the atmosphere as heat. This relative reflectiveness, or albedo, also plays a role in keeping cities warmer than the surrounding countryside as dark roofs and large areas covered with asphalt – surfaces with low albedo – contribute to what is called the urban heat-island effect.

Methane is more efficient than carbon dioxide at trapping heat. But the atmosphere contains much less methane than CO2, which stays in the atmosphere much longer. It takes about a decade for a fresh methane molecule to leave the atmosphere. A fresh CO2 molecule can stick around for millenniums.

Tyndall’s paper On the Absorption and Radiation of Heat by Gases and Vapours, and on the Physical Connexion of Radiation, Absorption, and Conduction in 1861, was the first to establish, if shakily, that CO2, water vapor, and other gases had radiative properties, although others had speculated similarly before.