From the website of Office of the Prime Minister’s Science Advisory Committee
Climate change
Article posted on Thursday, August, 13th, 2009 at 10:04 am
The world’s climate is influenced by a number of factors interacting in very complex and not entirely understood ways.
Over the last million years there have been periodic shifts in the temperature of the planet initiated by changes in the
orbit of the earth around the sun and in the tilt of the earth’s axis of rotation. These changes have led to periods of
global warming and global cooling – the more recent of the latter are termed the Ice Ages. There are also shorter-term
fluctuations brought about by a number of factors, including linked atmosphere-ocean changes with an irregular period of
several years (El Niño and La Niña events) and sporadic changes brought about by major volcanic eruptions. Global
warming does not mean that every part of the globe changes temperature to the same degree or rate.
Measuring global temperatures over time is complex, but there is a general agreement that the world is experiencing an
overall warming trend (with year-to-year fluctuations superimposed). The warming trend over the past 50 years is nearly
twice as great as that over the previous 100 years. These escalating temperature changes have been reflected in a number
of environmental and biological changes. These include rises in globally averaged sea level, shrinking of summer Arctic
sea-ice extent, losses from the Greenland and Antarctic ice sheets, retreat of mountain glaciers, poleward and upward
shifts in the range of some plant and animal species, and earlier timing for some species of spring events such as
leaf-unfolding, bird migration and egg-laying. That this is happening is not contentious.
This change in temperature is different in nature to past temperature changes. In particular, carbon dioxide
concentrations are rising in advance of, rather than as a result of, the warming trend.
The vast majority of the world’s climate scientists consider it very likely, based on several lines of evidence, that
the current warming trend is of human origin and is associated with increased production of the so-called ‘greenhouse
gases’ as a result of fossil fuel use, agriculture and deforestation. Humans had little impact on the global environment
until the advent of agriculture about 10,000 years ago. The flow-on effects of this agricultural revolution were
mutually reinforcing – domestication and farming of ruminant animals such as sheep, cattle and goats, clearing of
forests for pasture and crop growing, population growth because of greater food availability, and technological advances
leading to dependence on coal and other hydrocarbon fuels for utilisable energy.
Increased concentrations of the greenhouse gases trap heat (ultimately of solar origin) within that part of the
atmosphere closest to the earth. The major greenhouse gases emitted from human activities are carbon dioxide, methane
and nitrous oxide. Carbon dioxide has a very complex and slow cycle such that once concentrations in the atmosphere
increase, some of this increase will remain there for millennia. The other two gases have much shorter cycles, and thus
their atmospheric concentrations could be changed quite quickly. Warming of the atmosphere also leads to an increase in
water vapour content which further amplifies the warming, since water vapour is itself a strong greenhouse gas.
Ominously, the extent of human-induced global warming may be magnified by feedback effects that release even more
greenhouse gases into the atmosphere as the world warms (carbon dioxide is less soluble in warmer sea water, and more
methane may be released as the Arctic permafrost thaws) and cause less of the sun’s heat to be reflected by the melting
polar ice caps.
Other effects of increased carbon dioxide concentrations in the atmosphere include the gradual acidification of the
ocean, which could compromise the many marine organisms that build shells from calcium carbonate (such as molluscs,
krill and corals) and itself cause a feedback effect by slowing the uptake of carbon dioxide from the atmosphere.
Understanding the complexity of climate science requires the involvement of many scientific disciplines, and this
creates difficulties in reaching conclusions. There are unknowns, such as what will be the effects of altered cloud
patterns on climate as global temperatures increase. Nevertheless, there is a high degree of agreement among scientists
about the situation and the probable path ahead for our planet. Much of this agreement is encapsulated in the reports of
the Intergovernmental Panel on Climate Change (IPCC), which is the scientific group charged by the global community to
assess the state of understanding and integrate that understanding across these different scientific domains. The
periodic Assessment Reports of the IPCC – the fourth was in 2007 and the fifth will be completed in 2014 – inform
policy-making in the area of climate change. But confronting climate change brings controversy. The impact of climate
change is largely in the future, but to ameliorate it will require action in the present. The nature of the political
process makes it difficult to commit significant resources in the short term for long-term benefit. Furthermore, because
climate change requires global action, countries have difficulties suppressing national interest for united global
interest and there is a large amount of positioning between nations. The whole matter is compounded by the reality that
science cannot provide absolutely precise predictions about a future scenario for which there are no precedents. Action
on climate change therefore depends on a set of political decisions that in turn must be made on the current assessment
of the science and on the basis of scientific assessment of probability and risk.
While most scientists try to be detached and free from bias, they are never absolutely independent of their
philosophical and political views, and thus one would expect some strong and passionate debate. But in general there is
a high level of agreement on the trends and on expected future directions of change. Nevertheless, there are some
scientists, although few of these are active climate researchers, who dispute the generally held conclusions. Some
objections are based on faulty analysis or very narrow perceptions of what is important data. Other objections reflect
personal philosophical or political views. But there are some genuine uncertainties in the details of global warming,
just as there are in any other science. Such sceptical views are important, as they force the scientific community to
seek carefully for flaws in the analysis. A similar debate occurred about AIDS, where a minority of scientists
maintained for a long time that the disease was not caused by a virus. This view was manifestly wrong in the eyes of
most scientists, but nevertheless some distinguished scientists, albeit usually not experts in virology, took different
views until the science became irrefutable. The political consequences of this denialism had tragic results in some
African countries.
Unfortunately, because of the complexity of control of planetary temperature and because we are having to construct
predictions about the future rather than looking for a single present-day cause like a virus, efforts to calculate what
the global temperature might be in (say) 2090 involve estimates, and estimates always have a range – the likely upper
and lower values – and a level of uncertainty associated with them. For climate change, the uncertainty of the estimates
is further compounded by changes in human behaviour – will there be sustained efforts to reduce the emissions of
greenhouse gases and protect forests? Because some of this is happening outside the formal climate change initiatives,
these changes are already reflected in the more optimistic projections of the climate experts.
The scientific community has estimated the extent of the global temperature change at the end of the 21st century
compared to the end of the 20th century for a range of possible future scenarios if there are no policy-driven efforts
to reduce greenhouse gas emissions. Their best estimates are for an increase of between 1.8 and 4.0°C.
There is no specific global temperature rise above which we can say changes will be ‘dangerous’ and below which changes
will be ‘safe’. Any rise will have effects – indeed we are already experiencing some. However, the higher the rise the
greater the effect on our lives, and the scientific literature indicates many risks for more than a 2°C rise in global
temperature compared to pre-industrial conditions. The international view, supported by the actions of several
countries, has been to adopt a global warming limit of 2°C or below (relative to pre-industrial) as a guiding principle
for mitigation efforts, even though 2°C would still mean some changes in sea levels, in plant and animal ecosystems, in
agriculture and in environmental quality. In New Zealand, even this small increase will have effects on our agriculture,
coastlines and regional climates. The associated sea level rises will dramatically affect some of our Pacific Island
neighbours.
But this rise of 2°C is well below most estimates of what is likely to happen if the current pattern of emissions
production and rates of deforestation continue. In the absence of effective action, the mid-point of the IPCC estimates
is a global temperature increase of about 3.3°C, compared to pre-industrial conditions, by about 2090. For this reason,
there needs to be a global commitment to control the temperature rise. If the temperature rose by this amount then the
scenarios become quite scary in terms of changes in climate, flooding of low-lying areas, new patterns of infectious
disease, and reductions in the capacity of many parts of the world to support agriculture and therefore to support our
continued existence as we know it. New Zealand would not be immune from these changes.
Accordingly, the collective wisdom of the scientific community is that action is needed now. It is inherent in the time
scale by which emission targeting can affect temperatures that action sooner will have a greater ameliorating effect.
This means making decisions in the absence of absolute certainty. Certainty can never exist regarding the precise
magnitudes of temperature, rainfall and sea-level changes in advance of the periods you actually make the measurements.
We are dealing here with probabilities, and indeed dealing with probabilities is the normal business of science. Science
has done its best to reduce the uncertainty and now has a high level of confidence that something must be done now, and
that if nothing is done we will all suffer as global temperatures rise.
There is a remote possibility that if we did little or nothing then the temperature would not rise to unacceptable
levels. But we cannot gamble the future of the whole planet on the low probability of that occurring. We do many things
in life that are based on the balance of probabilities, for example we think it prudent to insure our houses and wear
seat belts in our cars not because we plan to have a fire or a crash, but rather because we are weighing the cost of the
insurance premium or the minor inconvenience of putting on the seat belt against the significant risk of damage to our
finances or ourselves if those events were to happen. It is the same with climate change – the collective wisdom of the
scientific community is that action is needed to address global warming because without action the potential risk to the
planet and ourselves is too high.
The problem that overlays all of this is one of economics. To reduce emissions and to protect forests, which absorb
carbon dioxide, has costs. The greater the degree of emission reduction required, the greater the cost. There are no
easy and economical ‘silver bullet’ solutions to prevent the carbon dioxide and other greenhouse gases produced by human
activity from ending up in the atmosphere, so substantial reductions in emissions are required. Global warming is indeed
a global issue, and much of the international political debate is over how to share the effort in reducing the rate of
temperature rise when some large emitters such as China and India wish to accelerate their economic development to reach
the living standards of countries such as Europe and the USA who have already had the benefit of industrialisation and
economic growth. Indeed, over the last year we have seen how central are the emerging economies to our own economic
health. Political arguments then flow as to the most equitable way of doing it – in absolute terms China and India are
large emitters, but when expressed per capita of population a different picture emerges and their emissions per person
are less than a quarter of those of the most developed countries. The global political community has yet to solve these
conflicting expectations.
New Zealand has a particularly unusual situation because about half of our emissions are derived from our farming
industry, as ruminants expel methane into the atmosphere. We have significant forests that offset our carbon emissions
and relatively low fossil fuel consumption, as we do not have much heavy industry. We are a long way from being able to
reduce emissions from sheep and cattle unless we reduce herd size, which would affect the heart of our economy. Active
research is starting to look at ways to change ruminant biology so they expel less methane; however while the research
is promising, it is still some way from application. So if we commit to reducing emissions by a certain percentage, say
20%, and we cannot change livestock emissions much we would have to have a 40% reduction in other emissions to meet that
target. This highlights the debate and dilemma over what emission target New Zealand should have. There is no scientific
answer to this question of what our target should be. For New Zealand is a small emitter by world standards – only
emitting some 0.2% of global greenhouse gases. So anything we do as a nation will in itself have little impact on the
climate – our impact will be symbolic, moral and political.
The ‘tragedy of the commons’ is a concept well understood in economics. If a pasture is owned by no-one but everybody
can graze their cattle on it without responsibility, then in time the common field will be available to no-one because
the field will have been overgrazed by some users at the expense of others. If that is the only field, then the
community can no longer support cattle. Such scenarios are not unusual and our species has fallen into this trap more
than once. On Rapanui (Easter Island) there were no trees left by the time the first European explorers arrived, and the
Rapanuians had thereby lost the ability to make canoes and to fish, except from the shore. Their lifestyle and indeed
their sustainability were forever radically altered.
The risk is that the consequences of a changing global climate will become another tragedy of the commons – if
collective action is not taken then everyone will suffer. The conundrum for the politician is real – how to achieve
collective action. That is not easy, because it means spending resources now to protect the world for future generations
and it is inevitable that every player is looking to protect their own short-term position. Unfortunately, science
cannot provide the complete answer – it can define the problem and investigate mitigating actions, but achieving a
solution is a matter of politics.
There is no easy answer – the science is solid but absolute certainty will never exist. As part of the global community,
New Zealand has to decide what economic costs it will bear and what changes in the way we live will be needed. We must
be involved. This is a global challenge, and a country like ours that aspires to be respected as a leading innovative
nation cannot afford to appear to be not fully involved. Indeed, such a perception would compromise our reputation and
potential markets.
ENDS