Climate Change Explained: Causes, the Greenhouse Effect, and Planetary Impacts

Climate change is the long-term shift in Earth's average temperature and weather patterns, distinct from natural day-to-day or year-to-year variability. According to NASA, the effects of this warming — rising temperatures, melting ice, and shifting weather patterns — are already measurable across the atmosphere, oceans, and ice sheets.

What causes climate change?

The underlying mechanism is the greenhouse effect: certain gases in the atmosphere trap heat radiated from Earth's surface instead of letting it escape to space. NASA explains that about 90% of this heat is absorbed by greenhouse gases and re-radiated back toward the surface, slowing the planet's heat loss. This effect is natural and necessary for life, but human activity has intensified it sharply since the Industrial Revolution.

NASA identifies four greenhouse gases most responsible for the intensified effect: carbon dioxide (CO2) from fossil-fuel combustion and deforestation; methane from livestock, landfills, rice farming, and fossil-fuel operations; nitrous oxide, mainly from fertilizer production and use, which NASA says has risen 18% over the last century; and chlorofluorocarbons (CFCs), industrial chemicals formerly used as refrigerants and propellants. Water vapor is the most abundant greenhouse gas but acts mainly as a feedback that amplifies warming rather than as a direct driver.

NASA states that industrial activity has raised atmospheric CO2 concentrations by nearly 50% since 1750. The WMO's State of the Global Climate Update 2025 report puts the 2024 figure more precisely: atmospheric CO2 reached 423.9 parts per million, 53% above the pre-industrial baseline of about 278 ppm, with methane and nitrous oxide also reaching record-high observed levels in 2024. The WMO also reported that the jump from 2023 to 2024 — 3.5 ppm — was the largest annual increase in the recent observational record.

NASA's causes page also addresses, and rules out, a common alternative explanation: solar activity. NASA says the solar energy reaching Earth has followed its natural 11-year cycle with no net increase since 1880, even as global temperatures have risen sharply over the same period, making the Sun an unlikely driver of the recent warming trend.

How much has the planet already warmed?

The WMO's State of the Global Climate Update 2025 report states that the January–August 2025 average was 1.42°C ± 0.12°C above the pre-industrial average, slightly below 2024's roughly 1.55°C ± 0.13°C — a dip the WMO attributes to the transition from El Niño to La Niña ocean conditions rather than a reversal of the long-term warming trend.

NASA's scientific-consensus page notes that about 97% of actively publishing climate scientists agree human-caused climate change is occurring, a figure drawn from the American Association for the Advancement of Science. NASA lists close to 200 scientific organizations worldwide, including national academies of science and bodies such as the American Geophysical Union and the American Meteorological Society, that hold the same position.

What is already changing

Both NASA and the WMO document impacts that are already underway rather than purely hypothetical. NASA reports that glaciers and ice sheets are shrinking and that heat waves are growing longer and more intense. Global sea level has risen about 8 inches since 1880, according to NASA, driven by melting land ice and the thermal expansion of warming seawater.

The WMO's State of the Global Climate Update 2025 report adds more recent detail. Sea level rose at 4.1 millimeters per year between 2016 and 2025, nearly double the 2.1 mm/year rate measured from 1993 to 2002. Arctic sea ice reached a record-low maximum extent of 13.8 million square kilometers in March 2025 — the lowest in the satellite record — while Antarctic sea ice reached a minimum extent of about 2.1 million square kilometers in February 2025. The WMO also found that the ocean absorbs more than 90% of Earth's excess heat energy.

What comes next

NASA's projections describe further sea level rise of at least one foot by 2100, and as much as 6.6 feet under high-emission scenarios, along with an Arctic Ocean that is expected to become essentially ice-free in late summer before mid-century if current trends continue. NASA also projects intensified hurricane rainfall, more severe droughts and heat waves in regions such as the U.S. Southwest, and wildfire-burned area potentially increasing two- to six-fold by 2050.

NASA notes, citing the IPCC, that some of the changes already underway — including continued sea level rise — are irreversible over the next hundreds to thousands of years, and that the effects will keep intensifying as long as greenhouse gas emissions continue to add to the atmosphere.