What is the exact sensitivity of the climate system to changing greenhouse gas, dust and aerosol loading in the atmosphere?

The IPCC AR4 has shown that, although we understand better and better how the climate system works, there are still considerable uncertainties in evaluating its future evolution. Not only due to uncertainties in human emissions, but also because of flaws in our physical and biogeochemical understanding of the system. The unpredicted recent evolution of Arctic sea ice or of the Greenland ice sheet are magnificent examples of such flaws.

Although everyone legitimally wants to understand the impacts of climate change (which drives many other global changes such as water resources, biodiversity, agriculture, extreme events, etc…), or wants to develop strategies of mitigation and adaptation to global change, there is little hope in doing a good job in the two latter aspects of global change if one does not improve our physical understanding of the system.

Climate sensitivity is the clear prerequisite to all other physical aspects such as tipping points, feedbacks, thresholds, etc… Solving this question allows one to basically improve the response to all other questions, from the physics to the mitigation, going through the impacts.

Do climate and biogeochemical models simulate past changes well enough that we can have confidence in their ability to tell us the effects of different options for the future?

It is pointless to make ever more detailed projections unless we have good confidence in the models – and right now they are still a ways away from being able to understand even the relatively recent past. We are far from being in a position to move away from a natural science agenda, as the Science editorial suggested!

What is the potential level of positive feedback that may come with the release of methane in the permafrost regions and continental shelves?

Given the potency of methane as a greenhouse gas, and also given the tremendous amount of methane stored both in the permafrost regions as well as the continental shelves, critical research needs to be done as to the level of positive feedback that may occur as some of this methane being released.

Massive releases of methane in earth’s past have played a pivotal role in dictating the direction and degree of climate change. While much attention has been given to carbon dioxide releases and sequestration, only a few scientists are currently studying methane releases going on in the thawing permafrost regions. Much more research needs to be conducted. The obstacles to doing this will be to move the focus of the press and policy makers from primarily carbon dioxide, to methane, as it is far more potent a greenhouse gas, and much more prone to significant positive feedback loops.

What will be the contribution to sea level rise of the ice sheets in Greenland and Antarctica over the coming century?

Sea level change is one the most outstanding issue in Earth System Science in terms of scientific and societal impact. The largest uncertainty in sea level projections is the rate of melting of ice sheets into the ocean in a warming climate. We are far from being able to make predictions of ice sheet evolution. Progress is urgently needed.

What are the spatial and temporal characteristics of natural climate variability?

The climate system exhibits internal variability on a broad range of timescales. However, the observational record is short, and the understanding of climate variability on decadal timescales and longer is therefore limited.

The detection and attribution of anthropogenic climate change requires a complete understanding of the spatial and temporal characteristics of natural climate variability. Furthermore, without such an understanding, we are not able to evaluate the ability of climate models to simulate past and present variability. This limits our confidence in the ability of the models to simulate future climate variability and change.

A critical focus of Earth system research over the coming decade should therefore be on combining observational and palaeoclimatic data to reconstruct natural climate variability over recent centuries and millennia.

What is the net effect of changing cloudiness on global climate?

The net effect of the present cloudiness is to clearly cool the global climate. Will the green house gas induced climate change strengthen or diminish the net effect of clouds on, e.g. surface temperature?

Can we quantify uncertainties in complex models of the complete Earth System?

The models we are using to base our judgements on are all imperfect. We must acknowledge this and attempt to address quantitatively how these uncertainties affect the judgements we are able to make on the basis of these models. Without uncertainty estimates (error bars if you like) any decision making will be arbitrary. With them it is more complex, remains subjective but at least can be justified and explained. This will be essential if the hard choices that we face are to be effectively communicated and acted on. Addressing uncertainty is central to all aspects of modelling, from the dynamic climate models to models of ecological systems, society and economics. Coupling such models makes uncertainty quantification even more essential. I believe this is a fundamental question to address before we can even start to answer specific questions about the Earth System.

What is the influence of trends in solar output on the atmosphere and oceans?

Because of the comparatively short period for which we have observations of the variations in the sun’s energy output, we have no reliable information of its longer-term trends and their probable effect on the earth’s temperature trends. Yet studies of indirect observations from Beryllium to numbers of sunspots suggest such and effect, despite present popular opinion derived from model experiments. How can we approach the problem in an objective manner?

How much and how quickly will methane be released from the polar regions?

There is more carbon locked up in methane (permafrost and clathrates) than carbon left on the planet in oil and gas. Methane is ~25 time more effective as a greenhouse gas so could have a massive impact on climate change. However we have little/no idea the rate of release of it either from the marine or the terrestrial envirment. Research is critically required now to understand the processed and then quantify the likely effects.

In terms of livelihood risks and planning challenges, is rainfall variability more significant than climate change?

Rainfall patterns typically vary in a basin. This variability presents major risks to farmers’ livelihoods and challenges to planners to create proper structures for water storage and conveyance. With climate change, the frequency of extreme events is expected to increase and make the impacts of variability more pronounced. It is therefore important to clarify the significance of rainfall variability and suggest approaches to document and deal with the challenges involved.