What Is Indirect Evidence of Climate Change?

As it relates to the study of climate, indirect measurements are measurements of things that are affected by climate and climate change. While direct measurements of climate (discussed in the first reading for this week) tell a story about changing temperature, indirect measurements tell a story about global changes that are happening because of changing temperature. Understanding how the Earth responds to warming is essential to understanding how our planet is changing, and how it may change in the future as warming continues. It is also an important part of ensuring the accuracy of climate models (discussed in the next reading), as these changes in the Earth can, in turn, have an affect the rate of climate change. Indirect measurements are also essential for the study of ancient climate conditions.

Many different types of scientists study how things in the natural world are affected by changes in climate. Some study how the natural world is changing in response to recent climate change. They do not measure the increasing temperature, but instead study the how that change is affecting other parts of the planet such as melting sea ice, bleaching coral reefs, and changes in the distribution of plants and animals.

Other scientists study indirect evidence to understand historic or prehistoric changes in climate. Scientists do not have methods for making direct measurements of ancient climate conditions. Instead, they study indirect evidence of climate change, known as proxy data. They interpret the evidence left behind such as fine layers preserved in ice sheets or lakebeds, and fossil animals, plants and pollen. For timeframes within the past millennium researchers also work with records kept by people such as harvest dates, records of tree flowering and lake freezing, and paintings and photographs of landscapes. Because the timing of documented seasonal events and landscape characteristics in pictures depend on climate, they provide essential clues. By combining data from various sources, scientists develop a broad understanding of past climate changes over hundreds, thousands, even millions of years for regions of the world.

Indirect Evidence of Modern Climate Change


Frequency (and severity) of extreme weather events: since climate is distinguished from weather by the long-term nature of the phenomena studied, the frequency with which unusual and extreme events occur is an aspect of climate. How often do droughts, floods, and blizzards happen? Is the number of tornadoes, hailstorms, or hurricanes that occur in a given year, decade, or century greater, less than, or about the same as the longer-term average? The severity of extreme events is closely related to this issue. If the number of category 4 and 5 hurricanes in a year increases, although the total number of hurricanes remains steady, we still recognize this as a shift in the climate. We may have the same number of floods or droughts in a particular decade, but if the droughts last longer or the floods inundate a larger area we presume that something about the climate has changed.

Satellite image of Hurricane Rita in 2005 (NASA)
gulf stream Ocean currents: the waters of Earth's oceans and seas play a huge role in carrying heat (or cold) from one part of the planet to another. Warm and cold water currents, and both surface and deep ocean currents, strongly effect global climate patterns. Climate scientists track sea surface and deep water temperatures, current directions and speeds, and salinity levels.

Gulf Stream current in Atlantic brings warm water (red) north. (NASA)

flower Phenology: numerous events tied to the changing seasons are measurable. The study of these phenomena, collectively, is called "phenology". Trees, of various types, bud in the spring and drop their leaves in the fall; we can note when and where each species does so. Insects also respond to changing seasons, emerging in the spring and mating at times tied to temperature and other aspects of the changing climate. We can note the first freeze of the winter and the last frost of the spring.
The timing of flower blooming in the spring is an indicator of seasonal change . (A.Pharamond)
florida possible sea level change

Sea level change: change in sea level is a measurable quantity that is closely related to climate change. During ice ages, fresh water that evaporates from the seas piles up on the ice caps over land and thus does not return to the ocean as runoff, so sea levels drop. During times of warmer climates the ice caps shrink, the meltwater flows back to the sea, and sea levels rise. Warmer water also expands and has a greater volume than colder water, amplifying the rise in sea level during warmer climatic episodes. Note that sea ice and icebergs displace the same volume as liquid water, so only ice on land (and not ice in water) alters sea level. Also, over very long timescales (many millions of years), the movements of continents can also alter global sea levels, as the relative proportion of shallow seas along continental margins compared with deep ocean basins varies, altering the overall volume of the gigantic "tub" that contains our oceans.

The red areas in this map of southern Florida (USA) indicate the areas that would be affected by one meter of sea level rise. (NASA)
Hudson Bay sea ice Ice and snow: seasonal and longer-term accumulations (or losses) of ice and snow impact climate in several ways. Large ice packs, polar caps, and glaciers act as "cold reservoirs" that can prevent areas from warming in the summertime. Bright white snow and ice cover reflects sunlight, inhibiting the warming effects of the Sun's rays on a landscape or the ocean's surface. Melting ice and snow in the spring or during warming trends swells rivers with runoff and injects fresh water into salty oceans, altering currents driven by density differences between fresh and briny water. Scientists measure sea ice thickness and geographic extent, the depth of the ice sheets in Antarctica and Greenland, and the size and movement rates of glaciers. They gage the depths of snowpacks, the albedo (brightness) of snow and ice, the calving rates of glaciers as they shed icebergs into the sea, and the melting rates of those icebergs as they drift into warmer waters.
Sea ice in Hudson Bay. Dark blue areas are open seawater. (L.Gardiner)

Indirect Evidence of Past Climates: Paleoclimate Proxies

tree rings

Tree-ring patterns provide data about past climates.
Credit: UCAR

Climate on Earth varies from place to place, but it also varies over time. Climate scientists study climates of the past to better estimate how our climate may change in the future. Quantitative records of climate, based on readings from instruments, go back just a few centuries. Written accounts of climate, such as records of droughts or floods or heat waves or cold snaps, go back a few thousand years. Earth's climate history, however, spans billions of years. In order to understand how climate has varied before the most recent few milennia, scientists seek clues from paleoclimate proxy records. Although there are no natural thermometers or rain gages that directly measure aspects of climates in the distant past, there are many natural phenomena that are profoundly influenced by climate. Temperature and precipitation influence the rate of growth of trees, and thus the thicknesses of the annual growth rings seen in tree trunks. Snow falls in the polar regions, laying down layers of various thicknesses that can be viewed in ice core samples - and trapping samples of atmospheric gases up to hundreds of thousands of years old within tiny bubles in the ice. Rivers swollen with meltwater from spring runoffs carry sediments downstream, depositing them in layers which tell us about the flow rates and thus the depths of snowpacks. These and numerous other natural phenomena provide us with clues about past climates. The data don't directly tell us about the climates of the past; recognizing this, we call these indicators proxy records, and must take care to tease out the climate data and to be cautious in our interpretations as we note the uncertainties involved. Nevertheless, such proxy records can tell us a great deal about the climates of the past throughout much of the incredibly lengthy history of our planet.

We look at paleoclimate proxies in much greater depth during the next week of this course.

The above was designed for an online course for educators from NCAR Online Education.

Last modified June 4, 2009 by Lisa Gardiner.

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