UK Climate Change Programme 2006 - JNCC - Defra

8
CHAPTER ONE
What is climate change and why does it matter?
The overwhelming majority of scientific
opinion supports the view that human
activities are changing the earth’s climate and
that likely future changes present a serious
threat to human society and the natural
environment.
The ten warmest years globally since
instrumental records began in 1861 have all
occurred since 1994. 1998 was the warmest
year and 2005 was almost as warm.
Without action to reduce greenhouse gas
emissions global temperatures are expected to
rise by between 1.4 and 5.8°C by 2100 and sea
level could also rise by between 0.09 and
0.88 metres compared to 1990 levels.
Such changes will have significant impacts on
human society and on the natural
environment, with increases in drought and
extreme rainfall and greater risks of abrupt
changes in climate.
The greenhouse effect and other
influences on climate
2. The temperature of the earth is determined by
the balance between energy from the sun in the
form of visible radiation (sunlight), and energy
constantly re-radiated from the earth to space in
the form of infra-red radiation. Sunlight passes
through the atmosphere with little direct
warming effect but it warms the earth’s surface
which in turn warms the atmosphere by
convection and the emission of infra-red
radiation, which is absorbed by certain trace
gases – the greenhouse gases. These return some
infra-red radiation back to the surface thereby
warming the surface further. An analogy is often
made with the effect of a greenhouse which
allows sunshine in through clear glass or plastic
that in turn stops the heat inside from escaping,
hence the term ‘greenhouse effect’.
Introduction
1) Visible and near-visible
radiation from the Sun
easily passes through the
glass and heats the ground,
plants, etc.
2) The ground, plants,
etc. act as a thermal
black-body radiator
and emit long-wave
infra-red (IR)
wavelengths
1. The earth’s climate has changed frequently over
the long periods of geological time in responses
to changes in the strength of the sun, the shape
and tilt of the earth’s orbit around the sun, the
position and shape of the continents and the
composition of the atmosphere. There is strong
evidence that greenhouse gas emissions from
human activities are now raising the earth’s
temperature and causing other changes in
climate. Emissions are projected to rise
significantly over the next few decades, and quite
likely beyond this, leading to significant increases
in global temperatures with profound risks for
the natural environment and human society
worldwide. This Chapter provides background
material on the nature of human induced climate
change and its likely impacts.
3) Most of the IR
radiation passes
back through the
glass and escapes
to space ...
4) Some IR radiation
is reflected by the
glass and is trapped
inside the
greenhouse causing
internal temperatures
to rise.
3. The greenhouse effect occurs naturally. Without
it the earth would be over 30°C cooler than it is
and largely uninhabitable. The main naturally
occurring greenhouse gases are water-vapour
(H 2
O), carbon dioxide (CO 2
), methane (CH 4
) and
nitrous oxide (N 2
O). Although water vapour
makes the greatest contribution to the
greenhouse effect, it has a short lifetime in the
atmosphere and its concentration is largely
determined by the temperature of the
atmosphere and not simply by emission or loss
rates. By contrast, the other three gases have
relatively long atmospheric lifetimes – about a