Monitoring Earth’s climate health involves tracking key measurements, much like a doctor assesses a patient’s well-being. These indicators provide insight into the global climate system, allowing researchers to identify trends and changes over time.
Earth’s Rising Temperature
The global average surface temperature serves as a primary indicator of Earth’s climate health. This measurement combines air temperatures over land with sea surface temperatures, collected from thousands of weather stations, ships, and buoys worldwide. To accurately track changes, scientists use temperature anomalies, representing the difference from a long-term average. This normalizes data for meaningful comparisons across regions and time.
The overall trend shows a significant warming over recent decades. The global average surface temperature in 2024 was 1.29 degrees Celsius (2.32 degrees Fahrenheit) above the 20th-century average. This rate of warming has accelerated, with each of the last four decades being warmer than any decade prior, dating back to 1850. The past decade, from 2015 to 2024, includes all ten of the warmest years ever recorded.
Atmospheric Carbon Dioxide
Atmospheric carbon dioxide (CO2) concentration is another fundamental indicator of Earth’s climate. Carbon dioxide acts as a greenhouse gas, trapping heat and warming the planet. Human activities, primarily the burning of fossil fuels, have amplified this natural effect.
Measurements of atmospheric CO2 are notably tracked by the Keeling Curve, based on continuous observations at the Mauna Loa Observatory in Hawaii since 1958. This curve reveals a steady increase in CO2 levels. In May 2024, the monthly CO2 concentration at Mauna Loa reached just under 427 parts per million (ppm), a new record. The global average CO2 for 2024 was 422.8 ppm. This represents a 50 percent increase compared to pre-Industrial Revolution levels and an annual rate of increase about 100 times faster than natural increases seen at the end of the last ice age.
Melting Ice and Rising Seas
The melting of Earth’s cryosphere, encompassing glaciers and ice sheets, provides a clear signal of a warming planet, directly contributing to global sea level rise. Since the early 1900s, many glaciers worldwide have been rapidly melting. Glacier loss has accelerated, with annual thinning rates nearly doubling from 36 cm in 2000 to 69 cm in 2019. This meltwater from glaciers accounted for 21% of global sea level rise between 2000 and 2019.
The major ice sheets in Greenland and Antarctica are also losing mass. Greenland is losing approximately 267 billion tons of ice per year, and Antarctica is losing about 136 billion tons annually. Meltwater from these ice sheets has contributed about one-third of the global average sea level rise since 1993. Global mean sea level has risen approximately 21–24 centimeters (8–9 inches) since 1880. The rate of global sea level rise has more than doubled, increasing from 1.4 millimeters (0.06 inches) per year throughout much of the 20th century to 3.6 millimeters (0.14 inches) per year from 2006–2015.
Sea levels are measured using a combination of tide gauges, which record water levels at specific coastal locations, and satellites, which use radar altimeters to precisely measure the ocean’s surface height from space.
Ocean Health
Oceans play a significant role in regulating Earth’s climate and exhibit their own vital signs. One key indicator is ocean heat content, which tracks the excess heat absorbed by the oceans. Since 1971, oceans have absorbed over 90% of the excess heat in the Earth system caused by rising greenhouse gas emissions. This absorption leads to thermal expansion of seawater, contributing to sea level rise. Ocean heat content is primarily measured using a global network of robotic Argo floats, which drift with ocean currents and collect data on temperature and salinity down to 2,000 meters deep.
Another important aspect of ocean health is ocean acidification. As oceans absorb excess atmospheric CO2, chemical reactions occur that increase the water’s acidity, lowering its pH. Prior to the 1700s, the average ocean pH was about 8.2, but it has since dropped to approximately 8.1. This seemingly small change represents a substantial increase in acidity, as the pH scale is logarithmic. Ocean acidification can negatively impact marine organisms and ecosystems, particularly those that form shells or skeletons.
Scientists monitor ocean pH by measuring factors like dissolved inorganic carbon and total alkalinity. This is done using various instruments, including sensors on ships and stationary buoys.