Climate evidence refers to data and observations scientists gather to understand long-term shifts in Earth’s climate. Unlike weather, which describes short-term atmospheric conditions over hours or days, climate represents average weather patterns over extended periods, typically 30 years or more. Scientists rely on multiple, independent lines of evidence to identify patterns and draw conclusions about these long-term changes. This ensures robust findings.
Direct Temperature Measurements
Direct temperature measurements collected globally over decades provide straightforward evidence of a changing climate. Since the late 19th century, a vast network of weather stations, ocean buoys, and satellites has recorded temperatures across land and ocean surfaces. Scientists combine these measurements to calculate a global average surface temperature, providing a view of planetary warming. Temperatures are compared to a long-term average to determine a “temperature anomaly.”
A positive anomaly indicates warming relative to the baseline, while a negative one signifies cooling. Data from NASA and NOAA consistently show a clear upward trend in this global average. The planet’s average surface temperature has risen by approximately 1.1 degrees Celsius (2.0 degrees Fahrenheit) since the late 19th century, with the most significant warming occurring in the past four decades. This sustained increase is evident in both land-based and ocean surface temperature records.
Diminishing Ice and Snow
Evidence from Earth’s frozen regions shows diminishing ice and snow cover across the globe, corroborating the warming trend. Mountain glaciers worldwide are shrinking, losing mass at accelerating rates. This retreat is observed globally, from the Andes to the Himalayas, indicating a response to rising temperatures. The meltwater from these glaciers contributes directly to global sea-level rise.
The massive ice sheets covering Greenland and Antarctica are also experiencing significant mass loss. Satellite missions, such as NASA’s GRACE and GRACE-FO, measure their decreasing mass. Greenland’s ice sheet alone has lost an average of 279 billion tons of ice per year between 1993 and 2019, while Antarctica lost about 149 billion tons annually during the same period. This loss of land-based ice is a major contributor to rising sea levels.
Arctic sea ice, which floats on the ocean, has seen a significant decline in both extent and thickness. Satellite observations show that the minimum sea ice extent in September has decreased by about 13% per decade relative to the 1981-2010 average. While the melting of floating sea ice does not directly raise sea levels, its reduction leads to a darker ocean surface, which absorbs more sunlight and further accelerates warming in the Arctic region.
Ocean Alterations
The world’s oceans are also undergoing significant changes as a direct consequence of the warming climate, impacting both sea levels and marine chemistry. Global sea level rise is a primary manifestation, driven by two main factors. First, as ocean water warms, it expands, known as thermal expansion. This accounts for approximately one-third of observed sea level rise.
Second, meltwater from glaciers and ice sheets, discussed previously, contributes the remaining two-thirds to rising sea levels. Global mean sea level has risen by about 20 centimeters (8 inches) in the last century, with the rate accelerating in recent decades. This rise poses risks to coastal communities worldwide.
Beyond rising levels, the oceans are also becoming more acidic, a process called ocean acidification. This occurs because oceans absorb excess carbon dioxide (CO2) from the atmosphere. When CO2 dissolves in seawater, it forms carbonic acid, increasing the water’s acidity and lowering its pH. This change in ocean chemistry makes it difficult for marine organisms, such as corals and shellfish, to build and maintain their calcium carbonate shells and skeletons, threatening marine ecosystems and food webs.
Atmospheric Greenhouse Gas Concentrations
Global warming is directly linked to increased atmospheric greenhouse gas concentrations. Scientists have tracked these gases, particularly carbon dioxide (CO2), for decades. The Keeling Curve, initiated in 1958 at Mauna Loa Observatory in Hawaii, is the most prominent record, showing a continuous rise in atmospheric CO2.
Before the Industrial Revolution, atmospheric CO2 concentrations were around 280 parts per million (ppm). As of May 2024, they have surpassed 420 ppm, an unprecedented level. This rapid increase correlates directly with human activities, primarily the burning of fossil fuels for energy, deforestation, and industrial processes.
While CO2 is the most abundant human-emitted greenhouse gas, other gases also contribute to warming. Methane (CH4), a potent greenhouse gas, is released from agriculture (livestock, rice cultivation), landfills, and natural gas systems. Nitrous oxide (N2O) comes primarily from agricultural activities, particularly nitrogen fertilizers. The cumulative effect of these rising greenhouse gas levels is an enhanced greenhouse effect, trapping more heat and leading to observed warming.
Geological Climate Archives
To understand if current climate changes are natural, scientists examine geological climate archives for long-term context. These archives use “proxy data”—indirect evidence preserved in natural materials—to reconstruct past climates over millennia. This allows comparison of present conditions with Earth’s climate history.
Ice cores, extracted from Greenland and Antarctic ice sheets, serve as climate records. As snow accumulates, it traps tiny air bubbles preserving samples of the ancient atmosphere. Analyzing these bubbles reveals past concentrations of greenhouse gases like CO2 and methane, while oxygen isotopes in the ice indicate past temperatures. Cores extending back over 800,000 years show that current CO2 levels are far higher than any seen during that period.
Tree rings provide annual records of climate conditions. The width and density of tree rings are influenced by temperature, precipitation, and other environmental factors during the growing season. Wider rings indicate favorable conditions, like warmer temperatures and adequate moisture, allowing reconstruction of past droughts, floods, and temperature fluctuations.
Sediment cores from oceans and lakes contain layers of preserved pollen, microfossils, and other organic matter. These materials reveal past vegetation, ocean temperatures, and sea levels, offering insights into climate conditions over vast timescales. These diverse geological archives collectively demonstrate that recent rapid increases in global temperature and atmospheric greenhouse gas concentrations are unprecedented in recent geological history.