1

The anthropocene: the current human-dominated geological era:
Human impacts on climate and the environment
Figure 1 shows me more than 70 years ago in the lap of my grandmother.
I have changed a lot, but so has much on planet Earth.
Human population has increased three-fold during mylifetime, reaching about six thousand million, with thelargest rise, 1.8% per year, after the 2nd World War. As
shown in the partial listing of Table 1, many humanactivities impact on earth’s environment, often
surpassing

naturewith

ecological,

atmosphericchemical and climatic consequences.

Figure 1

Anthropocene• During the past 3 centuries human population has increased tenfold to 6000 million• Cattle population increased to 1400 million (one cow per family)• Urbanisation has increased tenfold in the past century; almost half of the people livein cities and megacities• Industrial output increased 40 times during the past century; energy use 16 times• Almost 50 % of the land surface has been transformed by human action• Water use increased 9 fold during the past century to almost 1000 m3 per capita; 65 %for irrigation, 25 % industry, ~10 % households• Fish catch increased 40 times• The release of SO2 (160 Tg/year) by coal and oil burning is at least twice the sum of all
natural emissions; over land the increase has been 7 fold, causing acid rain, healtheffects, poor visibility and climate effects due to sulfate aerosol particles• More nitrogen is now fixed and used as fertilizers in agriculturel than is produced bynatural processes on land.
• Releases of NO to the atmosphere from fossil fuel and biomass burning is larger thanits natural inputs, causing high surface ozone levels over extensive regions of theglobe•Several climatically important ”greenhouse gases” have substantially increased in theatmosphere, eg. CO2 by 30 %, CH4 by more than 100 %.
• The production of chlorofluorocarbon gases has caused major changes in thechemistry of the stratosphere, especially ozone loss, which has been particularly large
in the spring over the Antarctic continent, the so-called ozone hole.

Table 1

Under the pressures of growing human populations, their need for food, water, and improved
living standards, land use changes have been substantial. For instance, during the period 1990
– 1997 the global annual rate of deforestation has been 0.5%/year with a maximum of

2
0.9%/year in south-east Asia. Human activities have especially accelerated after the 2nd World% Loss of
Total

War, several examples of which are shown in Figures 2a and 2b.

Figure 2a

Figure 2b

Starting especially by the end of the 18th century, the growing disturbance of earth’s natural
systems by human activities created a new geological era, which I have dubbed the“Anthropocene”.
In this paper we will discuss the consequences of human activities on the earth’s atmosphereand climate during the “Anthropocene”. Figure 3 shows the probably most cited diagrams on
the atmospheric consequences of human activities.

3

The upper curve shows the rise inatmospheric carbon dioxide concentrations
since the end of the 1950’s, as measured at
the Mauna Loa observatory on the island of
Hawaii. Embedded in the long term upward
trend are seasonal variations with lowest
values in the northern hemisphere during
spring and summer, reflecting uptake ofatmospheric CO2 by vegetation during thegrowing season, and highest values in fall
and winter due to the release of CO2 fromrotting vegetation. The lower two panels in
Figure 3

Figure 3 show the “ozone hole”, the drastic

loss of ozone which now regularly occurs in the stratosphere over the Antarctic continent inspringtime. The total ozone column depicted in the lower left, shows a total ozone loss sincethe 1960 – 1970’s by a factor of two in October. Two months after sunlight returns after thelong polar night, ozone is completely destroyed between about 15 and 22 km altitude, exactly
where naturally maximum ozone concentrations are found (right, lower panel). The ozone
hole came as a total surprise. It is due to a number of originally unexpected, positive
feedbacks, which start with the development of very cold temperatures, below about –80 °C,
during the south polar winter, causing formation of ice particles and production of very
reactive, industrial chlorofluorocarbon-derived, ozone-destroying chlorine atoms and chlorineoxide molecules by catalytic reactions on the particle surfaces in the presence of sunlightduring late winter/early spring. Although production of chlorofluorocarbon (CFC) gases nolonger takes place in the industrial world, because of the longevity of the CFCs, the ozone
hole will not heal before near the middle of this century. The ozone hole could only develop
by a series of positive (reinforcing) feedbacks, something to remember when we consider thehuman influence on earth’s climate system. Will there be surprises also possible in earth’s
climate system?

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In Figure 4 we depict estimatedchanges in radiative forcings
relativetopre-anthropocene,

A.D. 1750, conditions. The
radiative climate forcing due tothe growth in the atmosphericgreenhouse gases - CO2, CH4,
N2O,

the

CFCs,

and

tropospheric ozone- add up to
about 2.7 W/m2. This heating
Figure 4term is well known. Solar

variability constitutes a radiative forcing by up to about 10% of the anthropogenic greenhousewarming. Land use changes through urbanization and deforestation, and albedo changes by
ice sheet and glacier melting, also affect earth’s radiation budget by a similar amount, but do
not constitute dominating factors. The most important, but also most uncertain, negative
radiative forcings are related to the presence of aerosol particles (sulfates and organics) andclouds in the atmosphere. These terms are very uncertain, reflecting the large error bars.
Except for soot, atmospheric particles, in general, exert a negative (cooling) forcing on earth’s
radiation budget due to their propensity to scatter a fraction of the incoming solar radiationback to space. An additional important negative forcing is related to what is called theaerosols’ indirect effect, which is caused by the increased reflectivity of sunlight by clouds
due to enhanced number concentrations of aerosol, which also serve as cloud condensation
nuclei (CCN). As a consequence, for the same liquid water content, but with larger CCN
number concentrations, the polluted clouds will consist of a greater number of smaller
droplets, increasing the surface area of cloud droplets, in turn leading to enhanced
backscattering of sunlight to space by the clouds. Although it may sound contradictory, dirty
clouds are often brighter than clean clouds, except it they contain high concentrations of sootparticles. Furthermore, under such conditions, rain formation, which depends on coalescenceof cloud droplets to form bigger precipitating drops, also decreases, thus diminishing cloud
precipitation efficiency, producing longer-lived clouds and thus increasing cloudiness.
A tentative energy budget derived by the author and Prof. V. Ramanathan, of Scripps
Institution of Oceanography in La Jolla, California, for present atmospheric conditions
compared to pre-anthropocene conditions is shown in Figure 5.

5∆Greenhouse gas forcing (GHF) ≈ 2.7 W/m2
(since pre-industrial times)
Heating of the ocean≈ 0.3 W/m2

Increased upward IR(from warmer surface of earth)≈ 1 W/m2

Increased albedo effect≈ 1.4 W/m2(≈ 50% of GHF)

Direct heating of atmosphereby fossil fuel burning (1995)≈ 0.025 W/m2

Figure 5

As mentioned before, greenhouse radiative forcing is 2.7 W/m2. Uptake of heat by the oceansover the period 1955 to 1998 was 0.3 W/m2 on average. Enhanced upward flux of infrared
radiation at the “top of the atmosphere” due to the increase by 0.6 K in global average surface
temperature cools the planet by about 1 W/m2, assuming unaltered relative humidity in theatmosphere, which is a reasonable, but not totally proven, assumption. This implies an energy
surplus by 1.4 W/m2, which earth can only have gotten rid off by increasing its albedo via
enhanced backscattering of solar radiation by clouds and anthropogenic aerosols. This isabout half of the greenhouse infrared radiative heating, indicating the possibility of a doublingin infrared radiative forcing, if the atmosphere would become free from anthropogenic
aerosol. Future reductions in air pollution, which by themselves are desirable because of thenegative health effects of atmospheric particles, may, therefore, significantly enhance climate
warming by the greenhouse gases, constituting a major policy dilemma.
It is here very informative to compare the infrared greenhouse gas radiative forcing of 2.7
W/m2 with the direct heat release by humanity’s energy use, which is only 0.025 W/m2. Thus
production of the greenhouse gases, and not the direct heating of the atmosphere by energy
production, is the problem. A major effort to develop alternative energy sources other thanburning of fossil fuels is the only way to prevent excessive climate warming. Carbon dioxide
is not the only contributor to greenhouse warming. Together, other greenhouse gases haveprovided almost as much warming as CO2. Reductions in their emissions may be more easily
possible, for instance CH4 from fossil fuel handling, from cattle holdings, and rice fields,
N2O from the application of nitrogen fertilizers in agriculture, and NO from automobiles,
which results in a reduction of tropospheric ozone.
As shown in Figure 6, many complex and uncertain processes, with positive, as well as
negative, feedbacks determine earth’s climate.

6

In accordance with what is
expected from the growth inanthropogenic

greenhousegases, as shown in Figure 7,
global mean temperatures on
earth have increased since 1860
when direct temperature readings became possible.

Figure 6Thetemperaturerise since 1970 cancurrently only be
explained by theanthropogenic
greenhouse effect.
A significant part in
Figure 7

the rise in global

temperatures over the period 1910 to 1940, however, reflects contributions from other factors
than greenhouse gas emissions, such as heat release from the ocean, internal climate
variability, or increase in solar radiation. If the latter would be the main factor in climate
warming, as some claim, it would imply a very high climate sensitivity dTs/dQ = 0.4 °C/(0.3
W/m2) = 1.3 °C m2/W, where dTs is the increase in global average surface temperature anddQ the increase in radiative forcing in Watts per m2.
In its 2001 report, the Intergovernmental Panel on Climate Change (the IPCC) (Houghton etal., 2001) projected global average surface temperatures increases by 1.4 – 5.8 °C over thecurrent century, the large range being due to uncertainties, in particular in the modelling of thehydrological cycle (clouds, precipitation) and future scenarios of greenhouse gas and particle
emissions to the atmosphere.

The IPCC (2001) concluded “There is new and stronger

evidence that most of the warming observed over the last 50 years is attributable to humanactivities”. The warming would be accompanied by sea level rises of about 10 – 90 cm during
the present century. Thus, although uncertainties are large, current models, partially supported

7
by observations of surface temperature increases, indicate important climate changes, not tospeak about potential surprises as in the ozone hole case, briefly mentioned before.
To prevent major global warming, drastic reductions in greenhouse gas emissions are needed.
To stabilize the atmospheric concentrations of global CO2 and N2O, anthropogenic emission
reductions by some 60% are called for, a very difficult, if not impossible, task in a world inwhich the majority of people is still in a state of underdevelopment, as shown in Figure 8,
which depicts the current per capita use of carbon dioxide in metric tons per year.

Figure 8

Unfortunately, as shown in Figure 9, Nature does not come to the rescue. During the past
million years (the figure only shows the past 150.000 years), at any time when temperatures,
derived from ice core data, were high, also the atmospheric concentrations of CO2 and CH4
were highest, thereby actually strengthening climate
warming which is triggered by the behaviour ofearth’s orbits in the solar system.
A major uncertainty is the stability of earth’s
climatesystem

with

all

its

complications.

Especially sensitive may be the climates of thepolar regions, where largest climate warming isobserved and also calculated by models. Questions
have especially been raised about the stability of thethermohaline ocean circulations and the northAtlantic Gulfstream which transports heat fromlower latitudes to Europe.
Figure 9

8Although earth’s climate may be most vulnerable at high latitudes, the greatest future impact
by human activities on earth’s climate and the environment will be in the tropics, especially inrapidly industrialising Asia. Heavy air pollution now covers large parts of the tropics andsubtropics with high loadings of aerosol particles, containing relatively large amounts ofblack carbon, which produces the so-called Asian Brown Cloud. This is clearly visible fromspace, showing an extensive haze plume over India and flowing southwestward over the
Indian Ocean, towards the Maldives.
Besides soil dust, the aerosol consists of a
mixture of sulfates, organics, ammonia,
and black carbon (see Figure 10), producedby fossil fuel and biomass burning, and by
agricultural activities. Especially becauseof the strong absorption of solar radiationby the black carbon fraction of the aerosol,
Figure 10

major disturbances in the radiation budgetof the atmosphere over south, southeast andeast Asia occur during the dry season. As a result, substantial absorption of solar radiation
takes place in the atmosphere, reaching values of some 30 W/m2 over the Asian continent
during the dry season, which lasts from November to May. This is accompanied by a similar
decrease of the solar energy absorption at the earth’s surface. These numbers are about ten
times larger than the planetary anthropogenic energy disturbance at the “top of theatmosphere”. As a consequence of the enhanced energy uptake in the atmosphere anddecrease at the earth’s surface, the atmosphere will warm and the surface will cool, leading toa reduction in water evaporation at the earth’s surface, reduced cloudiness, less rainfall, and aslow-down of the hydrological cycle, which may also affect regions downwind of the polluted
regions. This Atmospheric Brown Cloud (ABC), caused by biomass burning and poor
technology is most pronounced over the Asian continent, but “brown clouds” are also present
over Africa, and South America. Their formation and persistence are favoured by the long dry
season which hinders the removal of the aerosol particles by precipitation. Because of thestrongly growing economic development of Asia, the ABC phenomenon will become anincreasingly major factor in anthropogenic regional climate change and rainfall
characteristics, especially over Asia. But there is a dilemma: combating air pollution by
reducing aerosol emissions, may very well promote a warmer climate.

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Finally, to finish where I began, Figure 11 shows me in 1999
together with my then two-year old grandson Jan Oliver.
I ask myself how the earth will be like during his lifetime which
will cover most of the twenty-first century. What is clear is thatin order to keep the earth habitable, major restrictions areneeded in the use of earth’s resources, below ground, at theearth’s surface and in the atmosphere. Mankind has a far way togo when it comes to a wise use of natural resources.
Figure 11

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Literature
Crutzen, P.J., Geology of Mankind: The Anthropocene. Nature, 415, 23, 2002.
Crutzen, P.J., and V. Ramanathan, Atmospheric Chemistry and Climate in the Anthropocene.
Chapter 4 in: Earth System Analysis for Sustainability, MIT Press, 2003.
Crutzen, P.J., and V. Ramanathan, The Parasol Effect on Climate. Science, 302, 1679, 2003.
Hofman, D.J., et al., Ozone profile measurements at McMurdo Station, Antarctica, During theSpring of 1987. J. Geophys. Res., 94, 16527, 1989.
Houghton, J.T., et al. (Eds.), Climate Change 2001; The Scientific Basis. Cambridge
University Press, 2001.
Keeling, C.D., and T.P. Whorf, Atmospheric CO2 records from sites in the SIO sampling
network, in Trends, a Compendium of Data on Global Change. Oak Ridge National
Laboratory, USA DOE, 2000.
Lelieveld, J., et al., The Indian Ocean Experiment, Science, 291, 1031, 2001.
Levitus, S., J.I. Antonov, and J. Wang, Anthropogenic Warming of Earth’s Climate System.
Science, 292, 267, 2001.
McNeil, J.R., Something New Under the Sun. Norton, New York, 2000.
Petit, J.R., et al., Climate and Atmospheric History of the Past 420,000 Years from the
Vostok Ice Core, Antarctica. Nature, 399, 429, 1999.
Rahmstorf, S., Thermohaline Circulation: The Current Climate, Nature, 421, 699, 2003.
Ramanathan, V., P.J. Crutzen, J.T. Kiehl, and D. Rosenfeld, Aerosols, Climate and theHydrological Cycle. Science, 294, 2119, 2001a.
Ramanthan, V., et al., The Indian Ocean Experiment, J. Geophys. Res., 106, 28371, 2001b.
Sateesh, S.K., and V. Ramanthan, Large Differences in Tropical Aerosol Forcing at the Topof the Atmosphere and Surface. Nature, 405, 60, 2000.
Steffen, W., et al. (Eds.), Global Change and the Earth System, Springer, 2004.
Tiempo. University of East Anglia, Global Climate 2000, 2001.