Tackling the threat of climate change is “the defining challenge of our age,” according to Ban Ki-moon, secretary-general of the United Nations. Last year, the influential Intergovernmental Panel on Climate Change (IPCC) published the most comprehensive study of climate change science to date which concluded that there was a 90 per cent certainty that the climate was becoming warmer and that human actions – in burning fossil fuels and chopping down forests – were largely to blame.

Meanwhile, the race is on to negotiate a successor to the ground-breaking Kyoto Protocol, the main provisions of which expire in 2012, with the issue set to come to a head at a crunch UN meeting in Copenhagen next year. What comes out of that meeting could determine the global response to climate change for decades to come.

Professor Jean-Pascal van Ypersele is one of the world’s most respected climate change scientists, a newly-elected vice-chair of the of the IPCC and professor of climatology and environmental sciences at the Université catholique de Louvain (Louvain-la-Neuve, Belgium).

To coincide with the second of the three part Financial Times Climate Change series, published today, Prof van Ypersele took questions from FT.com readers. A selection of the most thought-provoking are answered below.

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What should be the main strategy to tackle climate change globally?
Viktor O. Ledenyov, Ukraine

The IPCC is not in the ”should” mode, but highlights the consequences of different choices. The analysis of the literature on emission reduction potential in different sectors, from industries to transport and buildings, taking their cost into account, showed that all sectors and regions can contribute significantly to climate change mitigation. Both the technologies and policies to encourage them are available. The IPCC reports also show that achieving global sustainable development is associated with lower greenhouse gas emissions.

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What will be the effect when the world average temperature rises another 4-5 degrees celsius?
David Kay, unknown

The effects on human society and nature would be severe and widespread. Five degrees is about the change from current climate to a glacial period, but in the other direction. According to the last IPCC assessment report, this would lead to the extinction of more than 40 per cent of animal and vegetal species.

Food production would be seriously affected in many regions. Many more people would be affected by droughts, lack of water, and heat waves. Floods would also become more severe following more intense precipitation events, and if the warming is sustained during centuries, the sea level would rise by several meters, to roughly eight meters in 1000 years.

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I remain unconvinced that humans are the cause of (or, indeed, can fix) climate change. I am however amazed that while I see endless political initiatives aimed at changing human behaviour, I never see recognition of the fact that continued population growth will negate all the greenhouse gas reductions being engineered. Do you agree that population growth is the biggest single factor in the human contribution to climate change? Can any of the current initiatives work if we don’t also tackle world population growth?
Fred Perkins, London

There is some contradiction in your question, because population growth would have no influence on climate if humans were not emitting greenhouse gases. There are still large differences in emissions per capita between large emitters like the US and poor regions of Africa or India.

However, like anything on a finite planet, population growth cannot continue forever. Eliminating poverty is a key to stabilize the world population. This has been long recognized in the IPCC process, as it takes different emissions scenarios into account, reflecting a range of possible population growth and possible progress towards sustainability in all parts of the world.

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How can we be so sure that humans are the cause of climate change and not just a contributing problem to the normal changing of the world? History is full of ice ages and warm periods. Why is this any different?
Michael Ford, Sydney, Australia

Climate is affected by both natural factors (solar activity, volcanoes, changes in orbital parameters) and, more recently, by anthropogenic [man-made] factors (greenhouse gases, ”classical” air pollution, changes in surface reflectivity).

Nobody claims that humans are the only factor, but it is not possible to reproduce properly (with climate models) the climate of the last 100 years without taking into account the human factors. This led the IPCC to write that it was ”very likely” that the temperature increase since 1950 was mostly due to anthropogenic emissions of greenhouse gases.

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Assuming the worst-case-scenario in terms of future climate increase and the burning of fossil fuels as the primary cause, is there anything we can reasonably do that will have a significant impact over the next 50 years?
Luke Lea, USA

At that time scale, it is of primary importance to prepare the structural changes needed to avoid the build up of future emissions in the atmosphere, which would produce a climate warmer than at any time over the last two million years.

This is necessary to prevent the dangerous climate impacts that may occur in the second half of the century. Anything that contributes to reduce emissions will have some positive effect, but as gases accumulates in the atmosphere, that effect will not be immediate. Adaptation to the part of climate change that we will not be able to avoid is also very important for the next decades.

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Various parts of the Arctic show ice loss whereas most of the Antarctic is increasing in volume. Can you offer an explanation?
Graham White, unknown

Indeed, both Arctic sea ice which floats on the ocean and the Greenland ice sheet are melting. However, the sign of the volume change of the Antarctic ice sheet is at this stage, uncertain. The IPCC reports an Antarctic ice sheet contribution to global sea level rise between 1993 and 2003 from -1 (growing ice) to +5 mm/decade (melting ice). At lot of research is going on concerning the dynamics of ice in the Antarctic.

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Global temperatures, as reported by institutes such as the Hadley Centre, have levelled over the past nine years, in spite of increasing carbon dioxide. How is this consistent with the hypothesis of warming caused by greenhouse gases?
P E Binns, Edinburgh, Scotland

Temperatures actually broke new records since year 2000. It is only since 2 or 3 years that we did not experience higher global temperatures, but this is far too short to see climate change tendencies.

There are year to year natural fluctuations added over global warming. These short time scale changes have a number of causes such as the alternance of ”El Nino” / ”La Nina” events in the southern hemisphere and the 11-year solar activity cycle.

Both contributed to lower temperatures in the past year, but only temporarily (the cooling ”La Nina” recently ended). Unless there is a big volcanic eruption that injects lots of dust in the atmosphere, with a cooling effect lasting for a few years, it is almost certain that average temperatures will continue to rise in the next decade and beyond.

For more information, there is a discussion on short term climate variability and El Nino on www.realclimate.org. It compares the two main global average temperature time series (HadCRU from the Hadley Centre, UK and GISTEMP from GISS, USA).

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With sun-spot activity greatly diminished, might we be entering a period of global cooling?
Rory Brophy, Phoenix, Arizona

Continuous monitoring of total solar irradiance now covers the last 28 years. The data show a well-established 11-year cycle in irradiance that varies by 0.08 per cent from solar cycle minima to maxima, with no significant long term trend. In terms of radiative forcing (a measure of importance of a warming factor), solar activity changes provided 0.12 watts/m2 over the period 1750 - 2005, while the net effect of anthropogenic factors over the same period is 1.6 watts/m2.

Greenhouse gases are very likely to play a much more important role over the coming century than any changes in solar activity. It will be global warming for a long time.

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What can we do to save earth and stop global warming at the local/individual level apart from tree plantation and less consumption of fossil fuels?
Devansh Chaurasiya, India-Banglore

Less consumption of fossil fuels is key for climate change mitigation, especially if they are consumed without carbon capture and storage.

Tree plantation can help too, but we should remember that deforestation needs to be stopped as soon as possible, as it currently accounts for roughly 20 per cent of anthropogenic greenhouse gas emissions. Other areas where action can have a significant impact are agriculture, and to a lesser extent waste management, as these two sectors provide a large fraction of methane and nitrous oxide emissions.

IPCC does not provide a guide to action at the individual level, but highlighted in its last report that changes in lifestyle and behaviour patterns that emphasize resource conservation can contribute to developing a low-carbon economy that is both equitable and sustainable.

Examples include changes in consumption patterns such as meat consumption (as cattle both produces methane and increases food production requirements), building occupant behaviour, transport demand management through urban planning (that can reduce the demand for travel), and management tools in industry (staff training, reward systems etc) which can help reduce greenhouse gas emissions.

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Would it not be sensible to establish an internationally funded body with multi-billion dollar resources to identify, for the common use, technology to fix the problem of CO2 pollution?
Nicholas Brown, unknown

The IPCC has identified and assessed in its reports the technologies and policies that would allow ”fixing the problem”. A significantly increased international effort to implement those technologies and policies in favour of sustainable development, climate change mitigation, and adaptation is now needed, and would certainly have important benefits.

The climate negotiations under the UN Framework Convention on Climate Change have stimulated many coordinated international R&D and technology activities already. These cover, for example, the hydrogen economy, carbon sequestration, generation IV nuclear systems, renewable energy (which, by the way, will be the subject of a Special IPCC Report in 2010) and energy efficiency, and clean development.

The IPCC noted that additional work is particularly needed to assist poor countries with technology transfers (the subject of an IPCC Special Report in 1999) as these countries lack resources to mitigate or adapt to climate change. Creating funds is one thing, funding them at the appropriate level is much more difficult, and requires political will.

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If I understood the IPCC correctly then it will take up to 1,000 years before any changes initiated by man will change the rate of global warming. So why is the race on to negotiate a successor to the Kyoto Protocol that will take so long before it provides any benefits?
Marton, Zurich

No, no, no. It is true that there is inertia in the climate system, but is not that bad. It takes no more than 20-30 years to start seeing differences in the global temperature resulting from different emission scenarios. The differences are longer to come for sea level.

Two reasons why there is a “race on” to negotiate a new climate deal are: firstly, Kyoto extends only until 2012, and the carbon markets will crash if the ”cap” is not extended beyond; and secondly, Kyoto is a small first step (minus 5 per cent in 20 years, and only for industrialized countries) compared to the reductions that would be needed for most stabilisation targets.

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According to Dr James Hansen, [of the NASA Goddard Institute for Space Studies] the earth is in the imminent peril of dramatic climate change. If humanity wishes to preserve a planet similar to that on which civilization developed and to which life on earth is adapted, paleoclimate evidence and ongoing climate change suggest that CO2 will need to be reduced from its current 385 ppm to at most 350 ppm. Do you agree with Dr Hansen’s conclusions?
Antonio Telo, Lisbon

The IPCC is very cautious not to be policy-prescriptive. It tells policy-makers: ”if you do this (or do nothing), these are the likely climate consequences and impacts”. It is very clear from the last IPCC report that impacts of climate change on unique and vulnerable systems (such as polar and high mountain communities and ecosystems) have been observed already, with increasing levels of adverse impacts as temperature increase further.

For example, the IPCC assessed with medium confidence that approximately 20 to 30 per cent of species assessed so far are likely to be at increased risk of extinction if increases in global average warming exceed 1.5 to 2.5°C (relative to 1980-1999). As global average temperature increase exceeds about 3.5°C, model projections suggest significant extinctions (40 to 70 per cent of species assessed) around the globe. These are the cold facts.

If one feels that this risk of extinction, or the fact that ”hundred of millions of people [would be ] exposed to increased water stress” for a global temperature increase less than 0.5°C above the present value is not acceptable (but that is not for the IPCC to decide), then one can look at the concentration and emission scenarios that lead to a particular stabilisation temperature.

The IPCC reports that a CO2 concentration stabilized at 350-400 ppm leads to a global temperature increase that is 2-2.4°C (best estimate) above pre-industrial at equilibrium.

To achieve that goal, global CO2 emissions need to peak before 2015, and be reduced by 50 to 85 per cent in 2050 compared to 2000. If, rather, you are happy, say, with a 3.2 to 4°C increase, then your global emissions in 2050 can increase by 10 to 60 per cent. Since I write here on behalf of the IPCC, I will not express my personal preference.

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