NOAA National Weather Service
What is Climate Change?
Climate change is a long-term shift in the statistics
of the weather (including its averages). For example,
it could show up as a change in climate normals
(expected average values for temperature and
precipitation) for a given place and time of year,
from one decade to the next.
We know that the global climate is currently changing.
The last decade of the 20th Century and the
beginning of the 21st have been the warmest period
in the entire global instrumental temperature record,
starting in the mid-19th century.
Why is the Climate Changing?
Natural variability
Climate change is a normal part of the Earth’s natural
variability, which is related to interactions among the
atmosphere, ocean, and land, as well as changes
in the amount of solar radiation reaching the earth.
The geologic record includes significant evidence
for large-scale climate changes in Earth’s past. An
example of this variability is shown in the plot below
of temperature data for the last 420,000 years,
derived from an Antarctic ice core.
Temperature changes in Antarctica determined from the deuterium
proxy measured in the Vostok ice core record. (Reference: Petit,
J.R., et al. 1999. Climate and atmospheric history of the past 420,000
years from the Vostok ice core, Antarctica. Nature 399: 429-436.)
Available at
Line plot of global mean land-ocean temperature index, 1880 to present. Individual years are plotted and the blue
line is the five-year mean. (Data and plot available from NCDC at

NOAA National Weather Service
Page 2
Human-induced change
Greenhouse Gases
Certain naturally occurring gases, such as carbon
dioxide (CO2) and water vapor (H2O), trap heat
in the atmosphere causing a greenhouse effect.
Burning of fossil fuels, like oil, coal, and natural gas
is adding CO2 to the atmosphere. The current level
is the highest in the past 650,000 years. The Fourth
Assessment Report of the Intergovernmental Panel
on Climate Change concludes, “that most
of the observed increase in the globally averaged
temperature since the mid-20th century is very likely
due to the observed increase in anthropogenic
greenhouse gas concentrations.”
What is being done to Study the
Effects of Climate Change
There are numerous potential effects of climate
change. Extensive research is being done around
the world – a good deal within NOAA – to determine
the extent to which climate change is occurring, how
much of it is being caused by anthropogenic (man-
made) forces, and its potential impacts. In some
of these areas, there is not a consensus among
scientists and in fact, there are often conflicting
points-of-view and studies. However, with further
research, no doubt many questions regarding impacts
will be resolved in the future. Potential impacts most
studied by researchers include the effects on sea
level, drought, local weather, and hurricanes.
Most of our current knowledge of global change
comes from General Circulation Models (GCMs).
At present, GCMs have the ability to provide us
with a mean annual temperature for the planet that
is reliable. Regional and local temperature and
precipitation information from GCMs is, at present,
unreliable. Much of the global change research effort
is focused on improving these models.
Where Can I Find More
U.S. Climate Change Science Program:
NOAA National Climatic Data Center site on
Global Warming:
NASA GISS recent research website:
Global Change Master Directory:
Intergovernmental Panel on Climate Change
(IPCC) Website: