Man-Made Climate Change Versus Natural Variability
Sam Atwood and Mark Seaman
December 2006
Could recent climate change be due to natural variability? In this short paper we look at some of
the key natural and human influences on climate variation and compare their effects with the
temperature trends of the 20th century. We conclude that natural causes alone cannot explain
recent changes in temperature for the following reasons:



The changes are too large or in the wrong direction from those that would be caused by
natural variability;
The changes are more rapid than could be explained by long-term natural variability, and
yet more sustained than could be caused by short-term natural events;
The changes occur on a global scale, unlike some previous patterns that occurred only
regionally.
Natural Variability
Earth’s climate varies over time due to a number of natural causes. The physical mechanisms by
which they act on climate are fairly straightforward and the effects can be seen in both direct
observations and proxy measurements of temperature.
Instrumental records of temperature exist for the last 150 years or so. Scientists have also
reconstructed a temperature record for the last two millennia based on proxy measurements of
phenomenon such as tree rings that are affected by temperature variation. Until the last one
hundred years, temperatures never increased or decreased over the course of a century by more
than 0.2 C. Figure 1 shows several reconstructed temperature series for the last millennium.
A primary cause of climate variation is fluctuation in the amount of solar energy reaching the
Earth. In recent decades, satellites have measured solar output variations of about .08% over an
11 year cycle, for a range of ±0.5 ºC.1 The red line in Figure 2 gives the estimated forcing over
the last thousand years, based on proxy measurements of solar radiance. During a period in the
late 17th century known as the Maunder Minimum, solar radiance is estimated to have been 3 to
5 W/m2 below its present mean of 1366 W/m2.
Volcanic activity also has an effect on climate. Volcanoes emit aerosols that can have a global
effect on climate if the eruption is sufficiently strong. For example, the 1992 eruption of
Pinatubo in the Philippines had a net radiative forcing effect of -4 W/m2 that year.2 The effects of
volcanic activity on climate diminish exponentially so that the effects are negligible after about
four years. To have a sustained effect on climate, therefore, there would have to be a sustained
increase or decrease in the frequency of eruptions. Scientists have not detected such a trend over
the last two hundred years. The blue lines in Figure 2 represent these forcings.
1
2
J.T. Houghton et al, eds., Climate Change 2001: The Scientific Basis (Cambridge, UK: IPCC, 2001), section 6.11.1.
Hougton et al (2001), section 5.2.2.8.
Man-Made Climate Change Versus Natural Variability
Page 2
Figure 1. Temperature anomalies from direct and proxy measurement of temperature.
The shaded band is a 95% confidence interval for the first Mann et al study. Source: Houghton (2001), section
2.3.2.2.
Figure 2. Forcings from volcanic activity (blue) and variations in solar radiance (red).
Source: Stott et al (2003).
Some early critics of global warming pointed to what is known as the Medieval Warm Period
and asked whether recent temperatures are actually significantly higher than during that period.
Man-Made Climate Change Versus Natural Variability
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From AD 950 to 1200 wine grapes were grown as far north as Britain, and the Vikings were able
to colonize a relatively ice-free Greenland. Yet this pattern was not a global change and may
have only received attention because it occurred in the North Atlantic, an area familiar to most
paleoclimatologists. Similarly, evidence of a Little Ice Age, from 1450 to 1850, can only be seen
in observations from Europe and not at the global level. Such regional patterns appear
throughout the climate record. When regional records from the last two millennia are combined,
there is no evidence of significant warming or cooling trends at the global level until the 20th
century.3
On longer time scales, global climate has varied by larger amounts. Changes in the Earth’s orbit
affect the amount of sunlight received and the distribution of incoming solar radiation. These
changes occur in cycles of approximately 19,000-24,000 years, 41,000 years, and 100,000 years.
The small but sustained forcings brought on by these cycles result in the glacial-interglacial
fluctuations that have dominated Earth’s climate over the last two million years. From peak to
trough of these cycles, typical temperatures have fluctuated from peak temperatures near that
seen in the mid-20th century to between 4 and 6 C lower during glacial maximums. During the
warming phase of the glacial cycle, temperatures typically increase by about 0.2 °C per century,
though there have been periods when the temperature warmed by as much as 7 °C in a few
decades.4 Thus, the Earth has experienced natural swings in temperature even more dramatic
than those of the 20th century, but these are associated with positive feedback mechanisms and
variability linked with the planet's emergence from a glacial period.
Man-Made Climate Change
Man-made climate change can be understood as climate variability and change which is not the
result of natural variations in the Earth's climate system. While there is some uncertainty as to
the exact extent of this contribution to climate change, there is a scientific consensus which
specifies the theory, general range, and evidence for its existence. The primary causes of
anthropogenic climate change are human impacts on the amount of atmospheric greenhouse
gases (GHGs), aerosols injected into the atmosphere, and changes in the way land is used
throughout the world. Research into these three sources has yielded physically based theories,
observations, and trends which point to the existence of human induced impacts on the planet.
Greenhouse gases are responsible for warming of the Earth's surface due to the greenhouse effect.
This process involves GHGs in the atmosphere absorbing and reemitting radiation (primarily
from the surface) which has the effect of increasing temperatures below the gas layer. The
implication of this theory is that with an increase in GHGs, the surface of the Earth will begin to
warm. There is a continual cycle of accumulation and removal of these gases in the atmosphere
which had reached an approximate equilibrium over the last several millennia. A human caused
addition to this balance would serve to increase the GHG concentration in the atmosphere and
warm the surface of the earth.
This process has been witnessed in the atmosphere since about 1850 when humans began
emitting enough GHGs into the atmosphere that concentrations began to change. Carbon dioxide
(CO2) concentration in the atmosphere is commonly used as a proxy for anthropogenic changes
3
4
P.D. Jones and M.E. Mann, “Climate over Past Millennia,” Reviews of Geophysics 42 (2004) RG2002.
Houghton et al (2001), section 2.4.3.
Man-Made Climate Change Versus Natural Variability
Page 4
in GHGs because it has the largest effect on greenhouse forcings from gases emitted as a result
of human activity. The amount of CO2 in the atmosphere has been increasing since about 1850
and has been linked to human sources. Studies have shown that the Earth's equilibrium climate
sensitivity is between 1.5 C and 4.5 C for a doubling in CO2. This means that if humans double
the amount of CO2 in the atmosphere, the surface temperature will eventually come back into
equilibrium at a temperature between 1.5 and 4.5 degrees higher. Models project equilibrium to
be reestablished at a higher temperature after several centuries, with the 60% of the change
taking place within 25 to 50 years.5 Since the middle of the 19th Century, the concentration of
CO2 has increased from about 275 parts per million (ppm) to around 370 ppm today, while the
temperature has seen an increase between 0.6 to 0.9 C.6
Aerosols are small particles in the atmosphere which change the way radiation is reflected and
absorbed. The effect of sulfate aerosols emitted by humans is still being researched and has not
yet been well quantified. It is known to cause both direct and indirect forcings on the climate
system which affect temperatures at the surface and at different heights in the atmosphere. The
net effect is a cooling of the surface along with possible changes in atmospheric circulation and
the hydrologic cycle. However, aerosols will also have shorter lifetimes in the atmosphere than
GHGs, so their effects will be limited.
Land use forcings on the climate are due to the effect of different uses of land on the climate
system in general. Deforestation can decrease the CO2 removed from the atmosphere while
agricultural uses of land can chance the albedo of the surface and hydrologic cycle in an area.
While the effects of land use are still an area currently being studied, as much as 0.4 C cooling
could have resulted from this.
Combined Effect of Natural and Man-Made Forcings
Anthropogenic causes of climate change create a variety of effects on the Earth system, resulting
in both positive and negative forcings on the surface temperature. Models that ignore these
forcings and only consider the effects of natural variability cannot explain the rapid increase in
temperature during the 20th century. In fact, models that include both anthropogenic and natural
forcings more accurately simulate 20th century temperatures than do models that include only
one set of forcings.7
By itself, natural variability would cause the climate to cool slightly or change by at most 0.2 C
over the century, the maximum change seen over any of the preceding 19 centuries. Yet the 20th
century saw a 0.6 to 0.9 C rise in temperature. During the first part of the 20th century, measured
forcings from solar radiance and volcanic activity accounted for about half of the observed
increase, with anthropogenic forcings from greenhouse gas accumulation and sulfate aerosols
accounting for the rest. During the second half of the century, the forcing from greenhouse gases
implied warming at a rate of more than 1 C per century, and was primarily offset by a cooling
J. Hansen et al, “Earth’s Energy Imbalance: Confirmation and Implications.” Science 308 (2005), 1431-1435.
M.E. Mann et al, “Global-scale temperature patterns and climate forcing over the past six centuries,” Nature 392
(1998), 779-787; Jones and Mann (2004).
7
P.A. Stott et al, “External Control of 20th Century Temperature by Natural and Anthropogenic Forcings,” Science
290 (2000), 2133-2137.
.
5
6
Man-Made Climate Change Versus Natural Variability
Page 5
effect from increases in sulfate aerosols. As the aerosol forcing diminished towards the end of
the century, the net forcing increased as did the observed rate of temperature increase. The last
thirty years saw a positive forcing from increased solar radiance but this effect is less than half
that of the greenhouse gas forcing.8 (See Error! Reference source not found. THE
PRECEDING ERROR! ETC IS IN THE MS SENT TO ME. MAC.)
Figure 3. Natural and anthropogenic forcings during the 20th century
Forcing
Solar radiance
Volcanism
Greenhouse gases
Sulfate aerosols
Observed temperatures
1900-1949
temperature
trend
(K/century)
+0.29
+0.06
+0.27
-0.01
+0.76
1950-1999
temperature
trend
(K/century)
+0.17
-0.10
+1.08
-0.36
+0.72
1970-1999
temperature
trend
(K/century)
+0.48
-0.06
+1.10
+0.01
+1.52
Source: Stott et al (2003).
Conclusion
Natural forcings alone cannot account for recent temperature trends because the changes have
been larger, on a different timescale, and on a wider geographic scale than those forcings could
cause. On the glacial-interglacial timescale, temperatures are already at the high end of the
historical range; any significant temperature change due to natural variation would be expected
to be in the negative direction, concurrent with another glacial period. Volcanic activity acts on
too short a time scale to cause the recent sustained change in temperature, while fluctuations in
solar radiance can only account for smaller changes than have been seen. Finally, natural
temperature anomalies such as the Medieval Warm Period are not comparable to the recent
warming because they occurred on a regional scale and not globally. At the same time, models
that explicitly account for both natural and anthropogenic forcings simulate 20th century
temperature trends better than models that only include one set of forcings. This all leads to the
conclusion that it is likely that man-made climate change has been the primary cause of recent
trends in climate.
P.A. Stott et al, “Do Models Underestimate the Solar Contribution to Recent Climate Change?” Journal of Climate
16:24 (2003), 4079-4093.
8