What is climate change?

While natural causes such as solar or volcanic activity can lead to climate change, experts widely consider the global climate changes from the 20th century to the present day to result from human activity. Primarily, these activities involve the burning of fossil fuels, a process that releases carbon dioxide into the atmosphere.

The effects of climate change include extreme weather events such as droughts and heatwaves, changing ecosystems and impacts on human health and well-being. But mitigation of climate change is possible. Climate actions and sustainability efforts such as significant reductions in greenhouse gas emissions, advancements in renewable energy and progress toward the United Nations’ Sustainable Development Goals (SDGs) can alter future climate trajectories.

The Earth’s climate system has always been dynamic. Over the last 800,000 years, for instance, the planet has seen eight ice ages, with arctic ice sheets covering significant portions of North America, Europe and Asia. Climate science experts have determined that during the last ice age more than 11,000 years ago, global temperatures were about 6°C (11°F) lower on average than 20th century global temperatures.

Warmer periods follow ice ages and, according to NASA scientists, these climate cycles are linked to Milankovitch cycles: a series of orbital motions that affect how much light and energy the Earth absorbs from the sun.¹

There are other natural climate drivers, also known as “climate forcings”. Large volcanic eruptions qualify as climate forcings because sulfur dioxide emissions from eruptions can have a temporary cooling effect. Sulfur dioxide converts into sulfuric acid that condenses into aerosols in the atmosphere, reflecting the sun’s radiation away from Earth’s atmosphere. The US Geological Survey calls the 1991 eruption of Mount Pinatubo in the Philippines “a standout in its climate impact” as it generated the largest-observed sulfur dioxide cloud in history and cooled the Earth for three years.²

It is widely held by climate scientists that today’s largest drivers of climate change—and in particular of global warming—are human activities. The most impactful of these activities is the burning of fossil fuels, namely coal, natural gas and oil. Fossil fuel combustion generates greenhouse gas emissions, including emissions of carbon dioxide and methane.

Carbon dioxide and methane are called greenhouse gases because they form a barrier that traps heat in the Earth’s atmosphere in much the same way a greenhouse’s glass walls and ceiling trap heat indoors. This heat trapping, called the greenhouse effect, leads to global warming of the Earth’s surface.

Most greenhouse gas emissions are anthropogenic, which means humans and human activity cause them. According to the Intergovernmental Panel on Climate Change (IPCC), approximately 79% of all global greenhouse gas emissions stem from the energy, industrial, transportation and building sectors of the economy.³ Agriculture, forestry and other land use also generate a significant amount of emissions. For example, trees are natural stores of carbon, but deforestation—clearing away trees to make way for agriculture or other uses—releases that carbon into the atmosphere as carbon dioxide.

Today, we are focusing more than ever on how humans affect the climate, but human activities have been significantly changing the climate worldwide for over a century. To trace the origins of humanity’s measurable impact on Earth’s climate, look to the Industrial Revolution. As agrarian societies transformed into industrial ones by the late 1800s, fossil fuels increasingly powered the machinery and technology critical to the transformation. By the mid-20th century, carbon dioxide emissions totaled about 5 gigatons per year, then rose precipitously to 35 gigatons per year by the end of the century.

As a result of climate change, the Earth has experienced global warming, including the warmest decade on record between 2011–2020, according to the National Oceanic and Atmospheric Administration (NOAA). On the Earth’s surface, the average temperature is about 1.1°C (1.98°F) warmer today than it was before the industrial revolution.

This one-digit temperature increase is having profound effects on the planet. Scientists have found strong evidence that the temperature rise is contributing to:

• Extreme weather events
• Changes to natural ecosystems
• Harm to human health and well-being

As the Earth’s temperature continues to rise, dangerous heat waves are becoming more common. Heat can lead to more water evaporation, increasing the frequency of droughts. Drier conditions also lead to drier vegetation, which can fuel wildfires.

In other parts of the planet, more evaporation and increases in atmospheric water vapor is causing heavy precipitation and flooding. According to NOAA, experts expect that rainfall rates from tropical cyclones in particular to increase by 10%–15%.⁴

While extreme weather events take place over a discrete period of time, natural ecosystems are undergoing long-term changes and declines in biodiversity. For instance, higher temperatures are gradually melting Arctic Sea ice and glaciers, threatening wildlife ranging from polar bears to fish. Higher ocean temperatures in warmer climates like Australia and Florida are devastating coral reefs. Marine life is also at risk from ocean acidification, stemming from the absorption of carbon dioxide by seawater. 

Meanwhile, as some species suffer as a result of climate change, some invasive species are thriving, spreading beyond their original geographic ranges. For instance, milder winters in the United States might be helping an invasive vine called kudzu move to new areas and overtake native species, such as grasses and trees.⁵ According to the IPCC, climate change impacts on some ecosystems are irreversible.

Climate change causes or exacerbates extreme weather events and ecosystem changes that endanger many people every year. India’s seasonal monsoon showers, for instance, have become more intense in recent years—which scientists attribute to global warming—leading to hundreds of deaths from flash floods and landslides.

In Africa, a “new normal” of recurring droughts, floods and cyclones due to climate change might increase food insecurity in already vulnerable areas, according to a study by the International Monetary Fund.⁶ Overall, some 250,000 additional people might die each year due to climate change-related problems, ranging from disease to heat stress, according to the World Health Organization.

The effects of climate change also put livelihoods at risk. For instance, as the sea level rises—from melting ice sheets and glaciers—increasing floods and beach erosion threaten coastal tourism. Ocean warming and ocean acidification are hurting fish stocks and fisheries. Research by the IPCC predicts that continued global warming will cause declines in food production in regions around the world.

Scientists have been discussing climate change for well over a century. In 1896, Swedish physicist Svante Arrhenius wrote a paper predicting that changing levels of carbon dioxide in the atmosphere and the resulting greenhouse effect could affect Earth’s climate. Some forty years later, English steam engineer and amateur scientist Guy Callendar linked global warming to carbon dioxide emissions. And in the 1950s, Canadian physicist Gilbert Plass warned that anthropogenic carbon dioxide emissions were raising Earth’s surface temperature at a rate of 1.5°F per century.

Plass’s prescient warning notwithstanding, significant global efforts to address climate change didn’t begin until the late 1980s. The United Nations established the IPCC in 1988 while the UN General Assembly identified climate change as an urgent issue. Just under a decade later, in 1997, the Kyoto Protocol became the first international treaty to set legally binding targets for developed countries to cut greenhouse gas emissions.

The Paris Agreement of 2015 also included developing nations, extendeding emissions targets for all countries. The agreement aimed to prevent the global average temperature from rising more than 2°C (35.6°F) above preindustrial levels. Nearly 200 countries are signatories to the Paris Agreement.

In 2015, the members of the United Nations also adopted 17 SDGs, which included emphasis on adopting sustainable energy systems, sustainable forest management and lowering emissions.

In its sixth assessment report, issued in 2023, the IPCC confidently predicts that “deep, rapid and sustained mitigation, and accelerated implementation of adaptation actions” would reduce the adverse impacts of climate change on humans and ecosystems. The panel noted that since its fifth assessment report, issued in 2014, policies and laws on climate change mitigation have expanded. However, the IPCC concluded that those policies thus far are not likely to prevent global warming from exceeding 1.5°C (34.7°F) during the current century.

A key strategy for mitigating climate change is shifting away from fossil fuels and to clean, renewable energy. Renewable energy sources generate significantly lower greenhouse gas emissions than fossil fuel sources. Major sources of renewable energy include:

Other sources of renewable energy include bioenergy (energy produced from biomass such as crop waste and food waste), geothermal energy (heat energy from below the Earth’s surface) and marine energy (harnessing the kinetic and thermal energy of natural water flows).

Energy storage technology might be key to addressing the variability of renewable energy production in order for renewable energy sources to fully replace fossil fuels. According to the International Energy Agency, more progress in the growth of energy storage capacity is needed to achieve the agency’s goal of net zero carbon dioxide emissions by 2050.⁷

In addition to renewable energy technologies, other technologies and processes hold promise for climate change mitigation. For example, various technologies are being developed to remove carbon dioxide from the atmosphere and capture it from emissions sources. On an enterprise level, energy management programs are helping businesses monitor, control and optimize their organization’s energy consumption, helping lower carbon emissions while also reducing costs.