figure1. Earth’s global average
surface temperature has risen as
shown in this plot of combined
land and ocean measurements
from 1850 to 2012, derived from
three independent analyses of the
available data sets. The temperature
changes are relative to the global
average surface temperature of
1961-1990. Source: IPCC AR5, data from
the HadCRUT4 dataset (black), UK Met
Office Hadley Centre, the NCDC MLOST
dataset (orange), US National Oceanic
and Atmospheric Administration, and the
NASA GISS dataset (blue), US National
Aeronautics and Space Administration.
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Figure 1b. A large amount of
observational evidence besides
the temperature records shows
that Earth’s climate is changing.
For example, additional evidence
of a warming trend can be found
in the dramatic decrease in the
extent of Arctic sea ice at its
summer minimum (which occurs
in September), decrease in spring
snow cover in the Northern
Hemisphere, increases in the global
average upper ocean (upper 700 m
or 2300 feet) heat content (shown
relative to the 1955–2006 average),
and in sea-level rise.
Source: NOAA climate.gov | Yes. Earth’s average surface air temperature has increased by about 0.8 °C (1.4 °F)
since 1900, with much of this increase taking place since the mid-1970s (figure 1a).
A wide range of other observations (such as reduced Arctic sea ice extent and increased
ocean heat content) and indications from the natural world (such as poleward shifts
of temperature-sensitive species of fish, mammals, insects, etc.) together provide
incontrovertible evidence of planetary-scale warming. The clearest evidence for surface warming comes from widespread thermometer records. In some places,
these records extend back to the late 19th century. Today, temperatures are monitored at many thousands
of locations, over both the land and ocean surface. Indirect estimates of temperature change from such
sources as tree rings and ice cores help to place recent temperature changes in the context of the past. In
terms of the average surface temperature of Earth, these indirect estimates show that 1983 to 2012 was
probably the warmest 30-year period in more than 800 years.
A wide range of other observations provides a more comprehensive picture of warming throughout the
climate system. For example, the lower atmosphere and the upper layers of the ocean have also warmed,
snow and ice cover are decreasing in the Northern Hemisphere, the Greenland ice sheet is shrinking, and
sea level is rising [Figure 1b]. These measurements are made with a variety of monitoring systems, which
gives added confidence in the reality that Earth’s climate is warming. |