Table of Contents
Observing a Changing Climate
Climate change is not an abstract idea. It is observed through measurements, images, and physical changes across the planet. Scientists rely on many independent lines of evidence that, taken together, show that Earth is warming and that the climate system is changing rapidly compared with natural variations in the past.
This chapter focuses on the main types of evidence that climate is changing. It does not yet examine the causes or the detailed impacts. Instead, it explains how we know that change is happening now.
Rising Global Temperatures
One of the clearest indicators of climate change is the increase in global average temperature at the Earth’s surface. Temperature measurements come from weather stations on land, ships and buoys at sea, and satellites that observe the atmosphere.
Although there are year to year fluctuations, the long term pattern shows a strong warming trend, especially since the mid twentieth century. When scientists calculate the global average temperature for each year and compare it to a baseline period, the more recent decades are consistently warmer than earlier ones. The warmest years in the instrumental record have all occurred in the most recent decades.
Temperature records are adjusted carefully to account for changes in instruments, station locations, and observation practices. Independent research groups in different countries use their own methods, yet they all find a similar warming trend. This agreement among multiple datasets and methods is a powerful piece of evidence that the observed warming is real and not an artifact of measurement errors.
Melting Glaciers And Ice Sheets
Glaciers, which are persistent bodies of ice on land, respond to long term climate conditions. When the climate is warmer for decades, many glaciers lose more ice in summer than they gain in winter, and they retreat. Photographs taken many decades apart, combined with modern satellite data and field surveys, show that most of the world’s glaciers are shrinking.
In mountain regions, glacier fronts have moved uphill and backward, and glacier surfaces have become thinner. This retreat has been observed in the Alps, the Andes, the Himalayas, Alaska, and many other regions. Long term monitoring shows that the total mass of mountain glaciers, taken together, has decreased significantly over the last century.
Large ice sheets in Greenland and Antarctica also provide evidence of climate change. Satellite measurements of ice height and gravity show that both of these ice sheets, particularly the edges near the ocean, are losing mass. Greenland has experienced accelerated melting at the surface during warm seasons, as well as faster flow of some outlet glaciers into the sea. Parts of West Antarctica have also thinned and retreated. This loss of land ice contributes directly to global sea level rise.
Sea ice, which forms and floats on the ocean, is another sensitive indicator. In the Arctic, the area and thickness of sea ice have decreased sharply over recent decades. The minimum sea ice extent at the end of the Arctic summer has become substantially smaller compared to earlier satellite observations. In contrast, Antarctic sea ice shows more complex and regionally varied behavior, but the overall pattern of polar change, especially Arctic decline, is consistent with a warming world.
Rising Sea Levels
Global sea level is rising, and this rise is one of the most visible and measurable signs of climate change. Scientists use tide gauges along coasts and satellite altimeters over the open ocean to track sea level changes. Both methods indicate a clear upward trend over the past century, with an increase that has accelerated in recent decades.
There are two main physical reasons for this rise. First, as water warms, it expands. This process, known as thermal expansion, increases the volume of the ocean. Second, additional water enters the oceans when glaciers and ice sheets on land melt. These processes add up to a long term increase in the average height of the sea.
Local sea level changes can differ from the global average because land can rise or sink due to geological processes, and currents and winds can shift water around. However, when scientists combine observations from many locations and satellites, the global trend is upward. Coastal flooding that used to be rare is becoming more frequent in many low lying areas, providing real world confirmation of the data from instruments.
Changing Precipitation And Weather Patterns
Climate change is also evident in shifts in rainfall and weather patterns. Long term records show that in many regions, the timing, intensity, and distribution of precipitation have changed. Some areas receive more intense rainfall events, while others experience longer dry spells and increased drought risk.
Scientists look at indices such as the number of days with heavy rainfall, the length of dry periods between rain events, and seasonal changes in precipitation. Many places have seen an increase in very heavy rainfall events, which is consistent with a warmer atmosphere that can hold more water vapor. This can lead to more frequent or more severe flooding.
Patterns of large scale circulation in the atmosphere are also changing. For example, shifts in the position and strength of jet streams and storm tracks have been detected. These changes influence where and when storms occur, which regions become wetter or drier, and how often heatwaves and cold spells happen. While natural climate variability still plays a role, the long term trends point toward a climate system that is being altered.
Increased Frequency Of Heatwaves And Other Extremes
Extreme weather events provide another important line of evidence. Across many parts of the world, heatwaves have become more frequent, longer lasting, and more intense. Records for the highest daily or seasonal temperatures are being broken more often than records for the lowest temperatures.
When scientists analyze temperature data, they can examine how the distribution of daily temperatures has shifted over time. In many regions, the average has moved toward warmer values, and the tail of very hot days has become fatter, meaning that extremely high temperatures are more likely. This shift is a sign that the background climate has warmed, not just that natural variability is causing occasional hot years.
There are also observed changes in other types of extremes, such as heavy rainfall events and some types of storms. The evidence for each type of extreme varies by region and event category, and scientists are careful not to attribute every single event to climate change. Instead, they look at long term statistics and study how the likelihood or intensity of certain extremes has changed over time. These studies frequently find that climate change has made certain types of extreme events more probable or more severe.
Ocean Warming And Acidification
The oceans absorb a large amount of the excess heat in the climate system. Temperature measurements from ships, buoys, and autonomous floats show that ocean heat content has increased significantly over the last several decades. This warming is not only at the surface but also extends to deeper layers, indicating a lasting change in the energy balance of the planet.
Warmer ocean waters influence weather patterns, marine ecosystems, and sea level. For example, higher sea surface temperatures can contribute to stronger tropical cyclones, changes in fish distributions, and more frequent coral bleaching events. Coral bleaching occurs when corals, stressed by high temperatures, expel the symbiotic algae that provide them with much of their energy and color. Repeated or prolonged bleaching can lead to coral death and damage to entire reef ecosystems.
In addition to warming, the chemical composition of the ocean is changing because it absorbs a significant portion of the carbon dioxide from the atmosphere. This uptake of carbon dioxide leads to ocean acidification, a process where seawater becomes less alkaline. Measurements of ocean pH and dissolved carbon dioxide, along with data from marine organisms that build shells or skeletons from calcium carbonate, show that acidification is occurring and is happening faster than in known recent geological history. Although acidification relates directly to greenhouse gases, the important point here is that the measured changes in ocean chemistry are another independent sign that the Earth system is being altered.
Shifts In Ecosystems And Biological Indicators
Living organisms and ecosystems respond sensitively to changes in climate. These biological responses offer additional evidence that climate is changing. Observations from many regions show that plants are now flowering earlier in the spring, and growing seasons are often longer. In some areas, trees and other vegetation are moving to higher elevations or shifting their ranges toward the poles, following cooler conditions.
Animals are also changing their behavior and distribution. Many bird species are altering their migration timing, arriving earlier in breeding grounds. Marine species such as fish and plankton have shifted their typical ranges as ocean temperatures and currents have changed. Insects, including pests and disease vectors in some cases, are observed at higher altitudes and in regions where they were rare or absent in the past.
These shifts are documented through long term ecological studies, citizen science records, and monitoring programs. While individual species can sometimes adapt or move, the overall pattern of widespread, coordinated changes in many organisms is consistent with a large scale change in temperature and other climatic factors rather than small, local disturbances alone.
Paleoclimate Records And Context
To understand how unusual current changes are, scientists look beyond modern instruments and examine paleoclimate records, which provide information about past climate conditions. These records come from natural archives such as ice cores, tree rings, lake and ocean sediments, and corals.
Ice cores drilled from Greenland and Antarctica, for example, contain trapped bubbles of ancient air and layers of snow that compacted into ice over hundreds of thousands of years. By analyzing the gases and isotopes in these layers, scientists can infer past temperatures and greenhouse gas concentrations. Tree rings show how growing conditions changed year by year. Sediment layers in lakes and oceans preserve information about past temperatures, precipitation, and biological activity.
These diverse records reveal that while climate has varied naturally over long timescales, the recent rate and pattern of global warming, combined with the current levels of greenhouse gases, are very rapid compared to most known past changes. The paleoclimate evidence does not just show that climate can change, it also provides a context that highlights how exceptional the current trends are in both speed and magnitude.
Multiple Lines Of Evidence And Scientific Confidence
The evidence for climate change does not rely on a single type of measurement or one region of the world. Instead, scientists see consistent signals in air and ocean temperatures, ice and snow cover, sea level, weather extremes, ocean chemistry, and biological responses.
Different methods, instruments, and research groups independently confirm the main findings. Surface thermometers show warming, and so do satellite measurements of the lower atmosphere. Glaciers are observed to retreat in direct field surveys and in satellite imagery. Sea level rise is recorded by coastal tide gauges and by satellites observing the open ocean. Ecosystem changes are documented in many different environments by many different scientific teams.
The convergence of many independent lines of evidence, all pointing in the same direction, is what gives scientists very high confidence that climate change is real, ongoing, and measurable in the present day.
In summary, the world is warming, ice on land and sea is declining, sea levels are rising, weather patterns and extremes are shifting, oceans are warming and acidifying, and ecosystems are responding. These observations form the factual foundation for discussions about causes, impacts, and responses to climate change in the rest of this course.