Earth Systems

Greenhouse gases (GHGs) in the atmosphere allow heat from the sun to reach the Earth’s surface, but they trap a portion of the Earth’s outbound heat. That process acts much like a greenhouse does, so it is called the greenhouse effect. Most of the greenhouse effect is natural, primarily from water vapor, which takes up 2 percent of the air's volume, and natural carbon dioxide, which was at 280 parts per million before the industrial revolution. Without greenhouse gases, the Earth’s average temperature would be much cooler, -0.4°F (-18°C), rather than the pre-industrial average of 55.9°F (13.3°C) from 1850-1900.

However, human activities since the Industrial Revolution have significantly increased the rate at which greenhouse gases, such as carbon dioxide, methane, and nitrous oxides, enter the atmosphere. A major source of human GHG emissions is the burning of fossil fuels such as coal, petroleum-based liquid fuels, and natural gas for electricity. So long as human activities increase the volume of greenhouse gases in the atmosphere, the Earth’s average temperature will continue to rise as GHGs trap more of the Sun’s energy in the form of outbound heat from the Earth. This process is called “global warming.” The ocean has absorbed about 91% of the extra energy trapped by GHGs, with land warming, the melting of ice, and atmospheric warming accounting for about 5%, 3%, and 1%, respectively (AR6, SPM-14). In 2020, one of the hottest years on record, data from the National Oceanic and Atmospheric Administration (NOAA) showed the global average temperature was 58.77°F (14.87°C), that’s 2.92°F (1.62°C) above the pre-industrial average.

Throughout its long history, the Earth has warmed and cooled depending on how much sunlight the planet received. This varied naturally and over long periods of time due to subtle shifts in the sun’s orbit, how the Earth’s atmosphere or surface changed, or how the sun’s energy varied. Now, human emissions are dominating natural forces and the Earth is warming at an unprecedented rate.

“The rate, scale, and magnitude of anthropogenic [human-caused] changes in the climate system since the mid-20th century suggested the definition of a new geological epoch, the Anthropocene, referring to an era in which human activity is altering major components of the Earth system and leaving measurable imprints that will remain in the permanent geological record.” 

The Working Group I contribution to the Sixth Assessment Report (AR6), Climate Change 2021: The Physical Science Basis of the Intergovernmental Panel on Climate Change (IPCC). Chapter 1, pg. 17.

It’s the speed of the change that’s crucial. If the global average temperature increased a couple of degrees over millennia, it’s likely that humans could easily adapt. But if the changes take centuries, or even occur within a human lifetime, the results could be catastrophic. Since 1970 the global average temperature has been rising at a rate of 3.2°F (1.8°C) per century, compared to a long-term decline over the past 7,000 years of  0.02°F (0.01°C) per century. According to the latest IPCC report, the likely range of total human-caused global surface temperature increase from 1850–1900 to 2010–2019 is 1.4°F to 2.3°F (0.8°C to 1.3°C), with a best estimate of 1.9°F (1.07°C). The observed warming to 2010–2019 is 1.9°F [1.6 to 2.2°F] or 1.06°C [0.88 to 1.21] °C.” (That’s right, the IPCC’s best estimate suggests humans are likely responsible for 100%, if not more, of observed warming since pre-industrial times.) These rates of human-driven change far exceed the rates of change driven by natural forces.

Even abrupt geophysical events, such as the ending of ice ages, do not approach current rates of human-driven change. Scientists estimate that when glacial periods - known commonly as ice ages - have ended in the past, it has taken about 5,000 years for the planet to warm between 7.2°-12.6°F (4-7°C). Warming since the pre-industrial era has occurred at a rate about 7 to 11 times faster than during periods of ice age recovery and recent decades have warmed 13 to 22 times faster.

The rising concentration of carbon dioxide in the atmosphere due to human activities is increasing ocean temperature and causing ocean acidification, which occurs when the ocean absorbs carbon dioxide and forms carbonic acid. Both warming and acidification have been directly attributed to human greenhouse gas emissions.

The rate of ocean heat uptake has doubled since 1997, and this extra heat increasingly mixes into deeper layers, rather than remaining near the surface. Sea surface temperatures are much warmer now than they used to be – having risen 1.6°F (0.9°C) since the beginning of the 20th century, according to the latest IPCC report – and upper ocean heat content is setting records every year. This increase in ocean heat has led to a 5 to 15% increase in atmospheric moisture. As the moisture in the atmosphere increases, that provides more fuel for storms allowing them to grow more intense, last longer, and dump more precipitation. 

The uneven heating of the ocean — the surface warms more quickly than the deeper ocean layers — also causes the ocean to become more stratified, or layered between warm and cold water layers. Increased stratification reduces mixing of deep cold water with warmer water closer to the surface, which further increases the energy available to storms and the risk of intensification.

In addition to providing storms with more energy, another major consequence of ocean warming is sea level rise. As water warms, it expands, and between 1901 and 2018, global mean sea level increased by almost 8 inches (0.2m). Ocean expansion alone explains 50% of the sea level rise observed in recent decades (1971– 2018), though ice melt was the dominant factor from 2006-2018. Ocean expansion along with ice melt due to human-caused warming are directly responsible for most, if not all, the sea level rise observed since 1950, and are responsible for the recent acceleration of sea level rise.

The Gulf Stream System, also known as the Atlantic Meridional Overturning Circulation (AMOC), carries heat and carbon from the tropics into the North Atlantic, both at the ocean’s surface and deep down below the surface. It regulates the climate, mixes the oceans, and sequesters carbon. Human emissions are likely causing the system to weaken, or slow down, at an unprecedented rate, and it could cross a tipping point, which would have major consequences for global weather patterns.

Indicators and data suggest that human-caused warming has led to an “almost complete loss of stability” in the system that drives Atlantic Ocean currents. The slowdown of the Gulf Stream System is also one factor that’s likely contributing to unusually cold and fresh conditions in the North Atlantic.

Human-caused warming has led to drastic changes in the cryosphere, which includes continental glaciers (also called ice sheets), alpine glaciers, ice shelves, icebergs, sea ice, snow, and permafrost. Due to human emissions, the rate of ice sheet loss increased by a factor of four between 1992–1999 and 2010–2019. Ice sheet and glacier mass loss are now the dominant contributors to global mean sea level rise, overtaking ocean expansion from 2006-2018. Human-caused warming has also been driving the global retreat of glaciers since the 1990s and reductions in Arctic sea ice since the late 1970s.

The cryosphere plays an important role in regulating Earth’s temperature, and frozen regions are some of the most sensitive to climate change due to feedback loops that accelerate warming. Snow and ice, for example, help to keep the Earth cool by reflecting heat from the sun. When these bright white surfaces melt, darker land or ocean surfaces are able to absorb more sunlight, causing additional warming and melting. This is known as the ice-albedo feedback. As a result, the Arctic has warmed at two to three times the global rate in a process known as Arctic amplification.

Another important feedback in the Arctic is permafrost thaw. Permafrost is any ground that remains completely frozen—32°F (0°C) or colder—for at least two years straight. These permanently frozen grounds are most common in regions with high mountains and in Earth’s higher latitudes—near the North and South Poles. As permafrost thaws, microbes begin decomposing plant material in the soil. This process releases greenhouse gases like carbon dioxide and methane to the atmosphere, which in turn results in additional warming. In the Northern Hemisphere, near-surface permafrost area decreased by approximately 81,000 square miles per decade (or about the size of Kansas) during 1921-2005, largely due to human-caused warming. Abrupt permafrost thaw is one of the most frequently discussed “tipping points,”, or dramatic and irreversible changes to fundamental parts of the Earth system.

“From heat waves to record sea ice loss, the Arctic is yelling at us to pay attention. Unless we slow global warming by systematically reducing our greenhouse gas emissions, the chances of our first ‘ice-free’ Arctic summer will continue to increase. This rapid climate change in the Arctic will continue to have consequences for the entire Earth system.”

Zachary Labe, Postdoctoral Researcher at Colorado State University

Outside of the Arctic, another major concern is ice sheet loss and the potential for abrupt sea level rise. The Antarctic Ice Sheet, in particular, is an important indicator of climate change and a driver of sea-level rise. Some degree of irreversible loss of the West Antarctic Ice Sheet (WAIS) may have already begun. 

Ice shelves – permanent sheets of ice that float on the ocean and connect to a landmass – are another significant concern in the Antarctic from human-caused climate change. Ice shelves in the region are thinning because relatively warm water currents are eating away at them from below. They are also melting from the top as summer air temperatures rise abnormally high. Both these effects thin and weaken the ice shelves and, as they do, the glaciers they are holding back flow faster to the sea and their margins retreat.

Global warming doesn’t just change the likelihood of extremes; it alters the flow of weather patterns. Identifying the signal of climate change in a complex system such as the atmosphere is challenging, but a growing body of evidence shows that human GHG emissions are changing the behavior of planetary-scale atmospheric circulation patterns. At the most basic level, these patterns arise because the sun heats the Earth more at the equator than at the poles. Because Arctic amplification decreases the temperature difference between the poles and equator, a common feature of global warming’s impact on atmospheric circulation is the expansion of systems toward the poles, such as an increase in hurricane tracks into northern latitudes in the Northern Hemisphere.

One of the most well-established atmospheric responses to global warming is the widening of the Hadley circulation, or Hadley cell – an important circulation that transports energy from the tropics, northward in the Northern Hemisphere and southward in the Southern Hemisphere, to the subtropics. Humans are likely to have played a role in the expansion of the Hadley cell toward the poles since the 1980s, especially in the southern hemisphere. 

Climate change is also linked to increases in the frequency of weather patterns in the mid-latitudes with little or no movement—referred to as blocking patterns. These patterns often give rise to deadly weather events because they can cause weather systems to remain stationary for hours or days, intensifying extremes such as heat or precipitation. Arctic amplification and planetary wave stalling are two ways that climate change may be increasing the odds of blocking patterns.

Humans have directly impacted the plants and animals that make up the biosphere through changes in land and sea use, direct exploitation of organisms, pollution, the introduction of invasive species, and of course, climate change. Increasingly, climate change is exacerbating the impacts of the other drivers of change.

The response of plants and animals to climate change – as well as to the other human drivers of change – depends on the set of traits in a community. Climate change is shifting the set of desirable traits in regions across the globe, and plants and animals that are more flexible generally fare better. For example, animal species with the ability to burrow or those requiring a less specialized diet are more resilient to climate change. Among plants, species with shorter roots, less dense wood, and less ability to store water are more susceptible to drought than plants with the opposite traits. 

Many organisms are responding so quickly to human impacts that biological evolution is detectable within only a few years or even more quickly. Animals are getting larger beaks, legs and ears that allow them to better regulate their body temperatures as the planet gets hotter, with birds particularly affected. In many cases, the rate of climate change exceeds a species’ ability to easily adapt. One-third of all animal and plant species on the planet could face extinction by 2070 due to climate change, and climate change may have already impacted the distribution of nearly 50% of threatened mammals and nearly 25% of threatened birds.

Climate change is also altering the timing of important events in the annual life cycles of plants and animals – known as phenology. Across the world, many spring events are occurring earlier, and fall events are happening later, than they did in the past. Changes in the timing of these events – such as bud breaks for plants, breeding season, the first fall freeze or rainstorm – can have adverse effects on ecosystems, because species may respond to different environmental cues, resulting in a misalignment between species that rely on one another.

Rising ocean temperatures are causing extreme marine heat waves – or long periods of unusually high sea surface temperatures – which have become more intense and twice as frequent in recent decades due to human GHG emissions. Marine heat waves have immediate impacts on marine life and can cause ongoing shifts in the affected regions' food chain. In addition, the extra ocean heat can have knock-on effects that alter global weather patterns. Several recent marine heat waves had a human contribution of 100%, which means they could not have occurred without anthropogenic global warming. This includes the infamous "Blob" that lingered off the coast of the Pacific Northwest from 2013 to 2016 as well as the intense Southwest Atlantic heat wave in 2017.

Coral reefs are particularly vulnerable to climate change, and many that have been impacted by marine heat waves will never fully recover. Coral reefs respond to changes in water temperature, ocean pH, pollution and land use change near the coasts. Approximately half the live coral cover on coral reefs has been lost since the 1870s, with accelerating losses in recent decades due to climate change exacerbating other drivers. The live coral cover on coral reefs has declined by 4% per decade since 1990.