Climate Science 101
Is it weather or is it climate? Weather is the condition of the atmosphere at a given time and place. Climate, on the other hand, is a longer-term or "average" condition of the atmosphere that you might expect for an area.
Weather in Montana can vary considerably from day to day and month to month. This high variability is because we live in a semi-arid region with a cold continental climate. Generally, we receive relatively little precipitation on an annual basis. In Western Montana, the winters tend to be cold and wet with summers defined by hot and dry conditions. In Eastern Montana, the spring and summer tend to be wetter than the winter.
- Is it weather or is it climate? Though related, the distinction between these two
terms is often misunderstood. Look out the window and what do you see? Weather. Weather
is the condition of the atmosphere at a given time and place (3). Climate, on the
other hand, is a longer-term or “average” condition of the atmosphere that you might
expect for an area (1). Climate, as defined by the World Meteorological Organization,
is described using a minimum period of 30 year averages (3, a.k.a. a “climate normal”)
but can be more broadly described over hundreds, to millions, of years (1). Climate
may also be described by the severity and frequency of extreme events such as heat
waves, cold snaps, flooding, rains, blizzards, and severe droughts. Simply put, climate
tells what sort of clothes are in your closet, but the weather tells what to wear
on a particular day.
- A weather forecast is a prediction generated from current atmospheric conditions that
provides information about the likely temperature, wind, and precipitation over the
next six to ten days. In a similar manner, future climate projections provide information
on the likely condition of the climate conditions of a place over the next century
- Most of us have heard of “the greenhouse effect.” But what is it? The earth’s climate
and weather systems are powered by the radiant energy of the sun. About 30 percent
of the solar energy reaching the Earth is reflected back out to space, 20 percent
is absorbed by the Earth’s atmosphere, and 50 percent is absorbed by the Earth’s surface
(1, Figure 1). Some parts of the Earth receive more solar energy and some receive
less solar energy due to the Earth’s tilt on its axis of rotation and variability
in its annual rotation around the sun.
- Earth’s atmosphere helps to moderate the surface temperature (1). Incoming solar radiation
passes through the Earth’s atmosphere and heats Earth’s land and oceans (3). This
energy is eventually reflected or radiated back towards space. However, some of it
is absorbed by atmospheric gases and radiated back toward the Earth’s surface, which
further heats the surface and lower atmosphere. Known as the “greenhouse effect,”
this warming of our atmosphere acts like a blanket, making our Earth habitable (1).
Without this natural greenhouse effect Earth's average surface temperature would be
about 60°F cooler. In short, without the greenhouse effect, our planet would be too
cold to inhabit.
- Several trace gases, which account for less than a 0.1 percent of the atmosphere,
contribute to the greenhouse effect by absorbing and reradiating energy before it
escapes into space (5). The major long-lived greenhouse gases are carbon dioxide (CO2),
nitrous oxide (N2O), methane (CH4), ozone (O3), and industrial gases (1, 3). All of
these, except for industrial gases, are naturally occurring. These gases have a long
lifetime in the atmosphere, ranging from about 12 to 200 years or longer, and are
widely distributed throughout the Earth’s atmosphere (2).
- The concentrations of greenhouse gases in the atmosphere, which set the planetary
thermostat, have changed over the millennia, with relatively high concentrations being
associated with high average surface temperatures (Figure 2, 1).
- The Earth’s climate has changed throughout geologic history. For example, Montana’s
climate has varied substantially during the last 2.6 million years from extensively
glaciated to warmer periods between glaciation. Past changes have been associated
with natural causes such as: global plate tectonics (shifting of continental land
masses), variation in the sun’s energy output, changes in the earth’s orbit, global
volcanic activity, meteor impacts, and periodic changes in earth’s greenhouse gas
concentrations (Figure 2).
- Changes in global climate can occur on a timeframe as short as a 30-year period and
as long as a millennia. Volcanic eruptions are an example of a natural cause of short-term
climate change (6). Volcanic eruptions eject ash and sulfur dioxide (SO2) into the
atmosphere,which temporarily blocks a portion of the incoming solar radiation, resulting
in short-term global cooling (10 years or less, 1). Powerful eruptions that reach
the stratosphere (8-30 miles high), such as Mt. Pinatubo in 1991, can decrease the
global average temperature by nearly 1°F (0.5°C) for one or two years (1). El Niño
and La Niña climate pattern oscillations, as well as the Pacific Decadal Oscillation
(PDO), are other examples of natural drivers of short-term climate variability (1).
- Climate also changes on a much longer timescale, i.e. glacial periods and warm interglacial
periods that occur over thousands of years. Long-term climate change is caused by
changes in greenhouse gas composition and variability in Earth’s orbit with a resulting
change in the amount and distribution of solar radiation reaching Earth's surface
(7). Through proxy records, we know that Earth’s temperature has varied between 5-14°F
(3-8°C) from glacial to interglacial periods resulting in major changes in Earth’s
- Human activities are also known to influence climate. We experience human-caused warming
on a local scale when stepping from cool grass onto relatively hot concrete. This
is known as the “urban heat island” and is the result of land use change. Humans have
also increased the levels of greenhouse gas in our atmosphere (1). Sources of human-caused
increases in greenhouse gases include the burning of fossil fuels, industrial processes,
deforestation, and some agriculture practices (e.g. manure and soil management)(1,
- Since the industrial revolution, which began 200 years ago, Earth’s atmospheric concentration
of greenhouse gases has increased by more than 70 percent, far exceeding pre-industrial
levels (1). This increase in greenhouse gas concentrations traps more of the energy
being radiated by the earth that would otherwise have escaped back to space. We have
already seen that greenhouse gases set Earth’s temperature to habitable levels; however,
too much greenhouse gas in our atmosphere could result in undesirable changes to surface
climates that may challenge modern society. The observed increases of atmospheric
and surface temperature, ocean heat content, sea level, and melting of land and sea
ice are consistent with the patterns expected from rising levels of CO2 (1). Additionally,
changes in temperature increase atmospheric moisture and thus alter precipitation
patterns (Figure 3, 1).
- Given the Earth’s history of changing climate from natural causes, how is it known
that recent changes are not just part of the natural variability?
- The development of climate models has enabled investigation of the Earth’s climate.
Climate models are mathematical representations of past and future climate based our
best knowledge of the factors affecting climate (1, 3, 7). The wide variability in
model simulations comes from using an ensemble of many models (7). Even with the wide
variability, climate models that only include natural factors such as the Sun’s output,
Earth’s orbits, volcanic eruptions, and fluctuations such as El Niño and La Niña do
not fully explain recent documented changes to our climate (Figure 4). These simulations
yield little warming, or even slight cooling, over the 20th Century. Only when models
include human influences on the composition of greenhouse gases and land use change
are the resulting temperature changes consistent with observed changes.
- Widespread measurements of Earth’s surface temperature have been collected since around
1880; however, historic climate can be reconstructed from other historical evidence
and proxy data.
- Historical Evidence: accounts, artistic depictions, and photographs can provide evidence
on recent climate
- Proxy Data: a few examples include ice cores, lake sediment cores, coral, and tree
rings. Proxy data provides several strong examples of climate proxies from biological
and geochemical processes.
- Ice cores - contain dust and air bubbles that can be used to interpret the past past climate at the time the ice formed.
- Lake sediment cores - provide chemistry, pollen, charcoal, fossils and other evidence on the climate at the time the sediment was deposited
- Coral - the hard skeletons of coral contain forms of oxygen (isotopes) that can be analyzed to determine the temperature of the water when the coral was formed.
- Tree rings – tree ring records of ring-width, density, and isotopic composition provide records of annual climate conditions. The distance between one ring and the next can reflect past seasonal drought conditions, precipitation, snowpack, soil moisture, or temperature depending on species and growing location.
- 1. Intergovernmental Panel on Climate Change 2013. Climate Change 2013: The Physical
Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the
Intergovernmental Panel on Climate Change [Stocker, T.F., D. Qin, G.-K. Plattner,
M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)].
Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.
- 2. Blasing, T.J. (2014). Recent greenhouse Gas Concentrations. Carbon Dioxide Information
Analysis Center. http://cdiac.ornl.gov/pns/current_ghg.html.
- 3. Farmer, G.T. 2015. Modern Climate Change Science: An overview of today’s climate
change science. Springer International Publishing.
- 4. Karl, T.R., J.M. Melillo, & T.C. Peterson, (eds.) 2009. Global Climate Change Impacts
in the United States. Cambridge University Press.
- 5. Monson, R.K. & E.A. Holland. 2001. Biospheric trace gas fluxes and their control
over tropospheric chemistry. Annu. Rev. Ecol. Syst. 32: 547–576.
- 6. Langmann, B. 2014 . On the Role of Climate Forcing by Volcanic Sulphate and Volcanic
Ash. Advances in Meteorology, vol. 2014.
- 7. United States Global Change Research Program 2014. Climate Change Impacts in the
United States: The Third National Climate Assessment. Melillo, J.M., Richmond T.C.
& G.W. Yohe, (eds.).