The suggestion that humans are changing the
climate tends to evoke two main responses. One reaction is deeply skeptical:
"How could humans be a significant player in an epic cosmic scheme in which the
Earth's climate varies enormously between frozen ice ages, much cooler than
today, and periods like the Cretaceous when the average temperature was perhaps
20 degrees hotter than today?" The other common response is sympathetic: "Of
course the climate is changing: just look at how warm last winter was. Anyone
can see it's changing!"Climate change is a highly complex subject, spanning
several technical disciplines--from meteorology to climate modelling, from
economics to paleoclimatology. It's also highly politicized and contentious.
To help the average citizen navigate this debate, here are some frequently
> Is weather the same thing
No. We experience weather locally, at one place and at one
time. As everyone knows, weather can vary enormously over the course of one
year (100 degrees F or more) and between one region and another. Climatology
studies the average weather of the entire globe over centuries.
> How much
natural variation has there been in climate in the past?
variation has been large. Paleoclimatologists, who attempt to reconstruct the
climate's of past eras, have evidence that at times the Earth's average
temperature has been 10 degrees cooler than today and at other times 20 degrees
warmer than today. Many natural factors (operating on very different time
scales) are known to change climate: subtle variations in the Earth's orbit
round the sun; the positions of the continents as the tectonic plates move; the
output of the sun's light, the eruption of volcanoes, ocean patterns etc.
> How difficult has it been to discriminate human influence on climate from
It's been very difficult. Climatologists only have
comprehensive global temperature and rain fall measurements going back a
century. Using so-called "proxys" such as corals, tree rings, ice cores and
lake sediments, scientists have been able to reconstruct longer
records--back about 1,000 years. Over this period, the last 100 years appears
to stand out. (see graph) Since 1900, the Earth's average global
temperature has warmed by about 1 degree F--hardly noticeable to most of us,
but a rapid increase compared to most natural changes. Interestingly the shape
of this curve is not a straight line. Much of the warming occurred early in the
century, then it leveled out for three decades around mid century. This, it's
argued, shows that human greenhouse gas build-up is only one factor influencing
the climate. Other factors--for example, the changing output of the sun, other
pollutants like sulphate aerosols which have a cooling effect and,
volcanoes--also contribute to the aggregate effect.Why are many scientists and
policy experts worried about global warming, even though there is no "smoking
gun" demonstrating that serious human-caused global warming is underway? The
main reason people are concerned is the existence of a phenomenon called the
Greenhouse Effect. Trace gases in the atmosphere like carbon dioxide, methane
and water vapor can trap infrared radiation escaping into space. Without such
naturally occurring gases in our atmosphere, the Earth's average temperature
would be 0 degrees F instead of the comfortable 59 degrees F it is today.
However, too much CO2, can produce an inferno. (Venus is a good example of a
planet with an atmosphere made almost entirely of CO2.) Since carbon dioxide is
given off when fossil fuels like oil, coal and gas are burned, it follows that
more heat energy should be trapped in the climate system causing the global
temperature to increase.
> How much has the amount of CO2 in the atmosphere
Charles David Keeling's work on the mountain of Mauna Loa in
Hawaii established (see graph) that since 1957 the CO2 level has risen exponentially
and that the industrial CO2 diffuses throughout the entire
atmosphere in about 18 months. Samples from the Vostock Ice in the Antarctic
have extended the Keeling curve backwards in time, showing that CO2 has risen
one third since the industrial revolution, and it is higher than at any time
for the past 450,000 years.
> What part of this increased CO2 is
Most of it. Each year about 7 billion tons of carbon (which turns
into over 20 billion tons of CO2) are released by cars, factories and energy
production (see graph). About one half of this is absorbed by the oceans and the biosphere.
The rest stays in the atmosphere, joining other post-industrial revolution CO2
and remaining there for an average residence of 100 years. If things continue
as they are--with the developing world rapidly industrializing--then by 2100,
the world's annual emission of carbon will exceed 20 billion tons (70 billion
tons of CO2) and the amount (concentration) built up in the atmosphere will be
at least triple the pre-industrial level.
> Do we know just how this extra CO2
will change the climate?
Not really. Greenhouse gases like CO2 and methane
"force" the climate by trapping energy in the climate system. But it's just
part of a very complex system involving other forcings. For example, as
mentioned above, there are forcings like the sun's illumination which varies
over cycles of a decade or more. Or, volcanoes which spread ash into the air
and create a cooling effect. Other human pollutants like sulfate aerosols
(also produced by coal burning) actually cool the climate system by reflecting
incoming sunlight. But the biggest uncertainty is due to feedbacks which either
amplify (positive) or dampen (negative) the effects of a forcing. The principal
positive feedback is water vapor. Increased CO2 causes warming...leading to
more water vapor which is itself a greenhouse gas...leading to more warming.
Warmer ocean water bubbles out the dissolved CO2 which then leads to more
warming, more water vapor and so on. But there are negative feedbacks also.
Water vapor can form clouds which reflect incoming sunlight and thus have a
cooling effect. Finally, there are delays or inertias in the system, notably
the oceans which can absorb large amounts of heat for a time. The oceans,
therefore, act as a break on the climate system retarding change, delaying any
warming and cooling. To make things worse, the regional effect of a climate
change is not intuitive. In high latitudes, the increased water vapor might
fall as snow leading to major surprises as increased fresh water run off halts
or slows the so-called thermohaline circulation (driving the Gulf Stream) which
plunges far northern surface waters to the abyssal depths returning them to the
southern hemisphere. Interrupting this current might plunge the North Atlantic
into a deep freeze.
> To control greenhouse gas emissions, do we have to phase
out fossil fuels?
Sooner or later, yes. Because CO2 stays in the atmosphere
for about a century, simply freezing the annual emissions won't stop the
atmospheric concentrations rising. To stabilize the atmospheric levels of CO2,
be it at double, triple, quadruple preindustrial levels (or more), will
eventually involve phasing out fossil fuels entirely. This is extraordinarily
difficult.Fossil fuels are the basis of modern urban civilization. This highly
concentrated form of energy which can be used for electricity, heating and
transportation has enabled the evolution of large modern cities and a modern
infrastructure to ship people and goods all over the planet. Today, fossil
fuels are the primary source of 87% of the world's energy; the remainder coming
from biomass (firewood and charcoal), nuclear and hydoelectric. Fossil fuels
are not only concentrated and convenient, they are remarkably cheap.
> What are
the challenges in cutting our dependence on fossil energy?
biggest challenge is to cut our dependence at a time when the world's
population and energy demands are both rising rapidly. Over the next century,
world population will almost double to over 10 billion at least. And the
standard of living of billions of people will rise as they seek many of the
material benefits of the developed nations. This will require lots of energy;
rising from today's energy consumption rate of 10 terawatts (trillion watts) to
a rate of over 20 TW by the middle of the century and 40 TW or more by 2100.
> Will improvements in energy efficiency help?
Just a little. Switching
from high carbon sources like coal to lower carbon sources like natural gas
will also certainly buy the world time. The good news is that nations like the
U.S. are using energy more efficiently. In a typical American house, the
appliances are much more efficient than they were two decades ago: the
toasters, TVs, cookers and boilers. The bad news is there are many more
appliances like VCRs, computers, video games (many of which are left on); the
homes are larger; and the number of automobiles we drive has increased.
the only long-term solution is to replace fossil fuels, where will our energy
There are forms of energy which produce little or no carbon.
However, currently, there is no known source (with the possible exception of nuclear energy capable
of producing the massive amounts of energy the world will need. Hydroelectric
power is essentially carbon free, but the available rivers have all been
exploited and hydro will never contribute more a few percent of the total.
Nuclear energy is well established--440 plants produce 17% of the world's
electricity with no carbon dioxide or other air pollutants. But public concerns
about safety issues currently prevent a major expansion in this
source.So-called "new renewables" are continually mentioned as possible
solutions, but it's difficult to see how sources like solar, wind and biomass
can ever play a major role. Solar and wind are intermittent sources (the wind
only blows 25% of the time) which gather very small amounts of energy per
hectare. Biomass, essentially the construction of energy plantations, which
grow cellulose for burning or conversion into ethanol, is even less
concentrated. To produce 10 trillion watts (the current world power
consumption) using biomass would mean using 10% of the world's land surface
area (the area currently given over to agriculture.)
To sum up--there's no smoking gun to prove beyond doubt we are already changing the climate. But, if
current trends continue, the amount of CO2 entering and accumulating in the
atmosphere will go on rising. Given the vast reserves of fossil fuels, sooner
or later we will put enough CO2 into the atmosphere to force a climate change.
Just what this climate change will be, nobody knows. But once we have changed
the atmosphere, it will take hundreds of years to return to the way it was
before the industrial revolution.