Scientists measure climate change through a network of complementary systems: thermometers on land and at sea, satellites orbiting more than 1,000 kilometers overhead, robotic floats drifting through the deep ocean, and gas analyzers perched on remote mountaintops. No single measurement captures the full picture. Instead, researchers track several key indicators simultaneously, including surface temperature, greenhouse gas concentrations, ocean heat, sea level, and ice mass. Together, these paint a detailed and cross-verified portrait of a changing planet.
Surface Temperature Anomalies
The most widely cited measure of climate change is the global average surface temperature, tracked by institutions like NASA’s Goddard Institute for Space Studies for nearly 45 years. Rather than reporting raw temperatures, scientists calculate anomalies: how much a location’s temperature has strayed from a baseline average, typically drawn from 1951 to 1980. Anomalies are more useful than absolute readings because they stay consistent across elevation, latitude, and geography, making it possible to compare a station in the Sahara with one in Alaska.
On land, tens of thousands of weather stations in the Global Historical Climatology Network provide precise air temperature readings. At sea, ships and buoys capture surface water temperatures through the International Comprehensive Ocean-Atmosphere Data Set. Sea surface data were first combined with land records in 1995, improving the accuracy of global estimates. The result for 2024: the warmest year since records began in 1850, coming in at 1.29°C above the 20th-century average and roughly 1.46°C above the pre-industrial baseline of 1850 to 1900.
Greenhouse Gas Concentrations
The Mauna Loa Observatory in Hawaii is the global benchmark for monitoring atmospheric carbon dioxide. Situated high on a volcanic slope, far from cities and forests that could skew readings, it has tracked CO2 continuously since 1958. The current analyzer uses a technique called cavity ring-down spectroscopy, which measures how quickly a pulse of laser light decays inside a sealed chamber as CO2 molecules absorb it. The same instrument also measures methane.
Before air reaches the analyzer, it flows from intake lines atop a 38-meter tower and passes through a cold trap that freezes out water vapor, since moisture would interfere with the reading. To ensure accuracy, technicians calibrate the instrument against five standard gases with known CO2 values spanning the full natural range. Two additional “target tanks” are measured daily as an independent check. Nitrous oxide, another greenhouse gas, is measured separately using a gas chromatograph.
As of May 2025, monthly average CO2 at Mauna Loa reached 430.5 parts per million, an increase of 3.6 ppm over the previous May. For context, pre-industrial levels hovered around 280 ppm.
Radiative Forcing
Greenhouse gas concentrations translate into a physical quantity called radiative forcing: the extra energy, measured in watts per square meter, that the atmosphere traps compared to pre-industrial times. NOAA’s Annual Greenhouse Gas Index puts the 2024 total at 3.54 watts per square meter. Carbon dioxide alone accounts for 2.33 of that (about 66% of the total), methane contributes 0.57 (16%), and the remainder comes from nitrous oxide, CFCs, and other industrial gases. This number is essentially the planet’s energy imbalance, the fundamental driver of warming, and tracking it year to year tells scientists whether the problem is accelerating or stabilizing.
Ocean Heat Content
The ocean absorbs roughly 90% of the extra heat trapped by greenhouse gases, making it a critical indicator. Measuring that heat relies heavily on Argo, an international fleet of nearly 4,000 robotic floats scattered across the world’s oceans. Each float drifts with currents, periodically diving to 2,000 meters and rising back to the surface while recording temperature, salinity, and pressure along the way. Once it surfaces, it transmits the data via satellite.
Argo data have shown that the upper 2,000 meters of ocean hold the vast majority of the planet’s accumulated heat, with the 700-to-2,000-meter layer alone accounting for roughly one third of the warming in that range. A newer generation of Deep Argo floats extends sampling to 6,000 meters, covering nearly the full ocean depth. Before Argo launched in the early 2000s, ocean heat estimates relied on sparse ship-based measurements and carried large uncertainties. The float network has dramatically narrowed those error bars, improving projections of sea level rise in the process.
Sea Level Rise
Sea level is measured two ways, each capturing something different. Tide gauges, metal staffs or pressure sensors bolted to piers and coastlines, have tracked water height at fixed points for over a century. They measure relative sea level: the height of the ocean compared to the land next to it. That’s useful for local planning, but it conflates ocean changes with land movement, since coastlines can sink or rise on their own.
Satellite altimeters, operating from more than 1,000 kilometers up, solve this problem by bouncing radar pulses off the ocean surface and measuring the return time. They capture absolute sea level (the height of the ocean itself, independent of what the land is doing) with an accuracy of about 2 centimeters. Comparing the two datasets lets scientists separate genuine ocean rise from local land subsidence, which matters enormously for coastal communities trying to plan for the future.
Ice Sheet Mass Loss
Greenland and Antarctica hold enough ice to raise global sea levels by dozens of meters, so tracking their mass is essential. NASA’s GRACE mission (2002 to 2017) and its successor GRACE Follow-On (launched 2018) do this by measuring tiny changes in Earth’s gravity field. The missions use twin satellites flying in formation about 220 kilometers apart. As the lead satellite passes over a region with more mass (like a thick ice sheet), it accelerates slightly, changing the distance between the two spacecraft. That distance change, measured with extraordinary precision, reveals how much mass sits below.
When ice melts and flows into the ocean, the gravity signal weakens. Month-to-month gravity maps let scientists calculate exactly how many billions of tons of ice Greenland and Antarctica are losing each year, providing one of the clearest indicators of accelerating climate change.
Paleoclimate Proxies
Instrumental records only go back to 1850. To understand how today’s climate compares to conditions hundreds or thousands of years ago, scientists turn to natural archives called proxy data. Ice cores drilled from glaciers in Greenland and Antarctica trap tiny bubbles of ancient atmosphere, allowing direct measurement of CO2, methane, and other gases from as far back as 800,000 years. The ratio of oxygen isotopes in the ice reveals the temperature at the time the snow originally fell.
Tree rings provide year-by-year records of temperature and rainfall, with wider rings indicating favorable growing conditions. Coral skeletons contain trace metals and oxygen isotopes that reflect the ocean temperature in which the coral grew. Ocean and lake sediments, cave formations, and even pollen preserved in bogs each contribute their own piece of the puzzle. By stitching these records together, paleoclimatologists can place the current warming in a context spanning millennia, confirming that today’s rate of change is far outside the range of natural variability.
Extreme Weather Attribution
A newer branch of climate science, called extreme event attribution, asks a specific question: did climate change make a particular heat wave, flood, or hurricane more likely or more intense? Researchers run climate models twice, once simulating the world as it is (with elevated greenhouse gases) and once simulating a hypothetical world without human-caused warming. Comparing the two reveals how much climate change shifted the odds. Over the past 20 years, these studies have been combined with socioeconomic data to estimate not just the meteorological influence but the real-world damage, in dollars and lives, attributable to climate change. This makes climate measurement tangible in a way that global averages alone cannot.