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 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.

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.

How will the release of Siberian methane affect global warming? Can climate models predict its signature?

Because it’s a complicated process, and we can’t predict how much of the released gas may be re-absorbed by plants and the oceans, how quickly it will be released, or whether the sudden increase in methane will trigger an, as yet, un-predicted event. It is estimated that a Siberian thaw could push 500 billion tonnes of carbon dioxide gas into the atmosphere. The U.N.-sponsored Panel on Climate Change has published its estimate for global warming over the next one hundred years, producing a rise of between 3 and 11 degrees F. The addition of Siberian gas releases could change these predictions to 5 to 15 degrees F.

How will climate change impact groundwater resources?

Groundwater resources are the main source of drinking water throughout the world. The drivers to their future evolution are linked to most of the remaining uncertainties of climate modelling (future rainfall and intensity of it). Describing precisely the reservoirs geometry and the boundary conditions of aquifers is complex and limit our capacity to estimate the renewable water volume available.

How can we obtain non-biased probability distributions (i.e. error bars) for various aspects of Earth System model projections as climate sensitivity, sea-level rise or MOC strength?

Earth System models used for decadal and centennial projections are usually manually tuned in a high dimensional parameter space to fit current day’s climate. For physically and statistically reliable PDFs of climate quantities of interest at the end of this century we need methodical development and improvement of: automatic tuning processes, valid training data sets & methods, automatic initialisation & bottom up and top down validation attempts. Political decision methods are based on costs and benefits derived from these resulting PDFs and political action/inaction is decisive in this decade. We need to improve the quantitative aspects of the Earth System modeling enterprise substantially to justify massive welfare redistributions for the coming generations. The suggested technical development extends the questions concerning climate sensitivity and sea-level rise because it addresses general methodological difficulties in Earth System Modeling.

How will different landscapes on Earth react on future climate change?

It is important to find out about future climate change (especially by reconstructing the past), but I think it is even more important for the adaption to future changes if we know how the landscapes we are living in will react on it and how sensitive what kind of landscape is to climate change. Landscape means the natural or semi-natural area where people are living and which can be affected by direct and indirect changes in the hydrological and geomorphological system, e.g. (soil)erosion, vegetation loss or gain, desertification and so on, which all needs to be looked at in a holistic way, subordinated over and including different parts of earth system sciences considering various scales in time and space. The best way to find out about the future then is to look into the past and interpret and combine it with modeling of present landscape processes in different climate regimes. Then we might get a glance on how we have to or can adapt to different future perspectives and climate change however it will be, preserving human’s livelihoods.

How can local and regional environmental changes be scaled accurately and effectively to enhance the assessment of global changes, and vice-versa? How can we enhance the applicability of global predictions of biodiversity loss, water scarcity, climate change etc. to local and regional decision-making?

Environmental change occurs at varying temporal and spatial scales. Therefore it is imperative we understand how to utilize existing models and scientific understanding to create realistic predictions of biophysical thresholds and environmental consequences that governments and communities can utilize and comprehend. Furthermore, we need to understand how to relate the mitigation, adaptation, and conservation policies of a given locality to the Earth system. Knowledge of how to effectively scale human decisions and environmental change will significantly aid our ability in generating tangible action towards cooperative solutions.

How will polar climate respond to continued global warming?

Current climate models do not adequately simulate the behaviour of the climate at the poles (a) because they are based largely on tropical to mid-latitude processes, and (b) because they ignore the effects of changing ozone through time. Without improvements in polar climate models we shall be unable to provide any adequate projection of the decay of ice sheets and their contribution to sea level rise.

What is the role of the biosphere in regulating climate, and which systems and processes are most influential?

Climate models tend to include interactions with the biosphere quite generally, yet the ecological and evolutionary responses and feedbacks are potentially very significant but hard to predict. An obstacle is that we probably don’t know enough about the processes to develop better models until more relevant basic research has been undertaken.