How will the global water cycle evolve in response to global warming?

No element of the climate system has as much impact on society as the water cycle, yet we remain ignorant of the largest part of the water cycle, that over the oceans. The oceans are the main reservoir of free water on the planet, the source of nearly 90% of global evaporation and the site of ~80% of global precipitation. A mere 1% of Atlantic ocean precipitation matches the discharge of the Mississippi River. Water evaporates more readily from a warmer ocean, so an intensification of the water cycle is expected with anthropogenic warming. The signature of the water cycle within the oceans is in the distribution of salinity, which must be monitored in the future if we are to understand how the water cycle is changing. In addition, salinity influences ocean mixing and circulation and thus the ability of the ocean to absorb, store and transport heat and CO2. Can we initiate an observing system for upper ocean salinity that will help us to understand and predict the future evolution of the global water cycle? Can we develop a better understanding of the smaller terrestrial water cycle, where plants and drainage basins are responding to rising warmth and CO2? Can we understand the interactions between ocean, atmosphere and the high latitude ice sheets that are leading to increased melting and discharge to the ocean? The global water cycle is truly a central unifying problem for climate change, and of fundamental importance to society.

Which is the space weather for tomorrow?

The active Sun has an incontestable influence on our Planet and its environment. The effects of the solar-terrestrial relations are known as space weather. Forecasting the space weather conditions is crucial as they affect our life, via:

- Geomagnetic storms which are caused by impacts of coronal mass ejections on the terrestrial magnetosphere, leading to very rapid changes of the geomagnetic field. This can induce strong currents in the extended electric
conductors like high voltage lines or pipelines. The consequences are power outages and damaged transformers, or rapid corrosion of pipelines.

- Energetic protons and electrons that are produced by flares and coronal mass ejections, and which can damage the electronics of satellites, and endanger astronauts.

- Enhanced electromagnetic radiation, mainly X-rays from flares modify the upper atmosphere, which absorbs this radiation, is heated up, increasing the air density in the range of low satellite orbits. Satellites can deviate from their orbits due to the enhanced aerodynamic drag and eventually crash. This radiation can also change the structure of the ionosphere, which in turn can affect the short-wave radio communications, but also for navigation systems like GPS, since precise position requires exact modeling of the propagation of satellite signals through the ionosphere.

What factors determine the resilience of the full set of interacting ecosystem services that support human well-being and allow for adaptation to a changing environment?

This question requires interdisciplinary research among physical, biological and social sciences. It raises significant conceptual or theoretical issues, as well as significant needs for empirical research at global and regional scales. The question quickly gives rise to a host of important more specialized questions. Answers to this family of questions are relevant for applied questions of sustainability science.

Definitions: (1) Resilience is the capacity of a system to persist within thresholds or “guardrails”, adapt to changing circumstances, or transform to something new when the current mode of operation is unsustainable. (2) Ecosystem services are benefits that people receive from nature, such as provision of food and water, regulation of water flows and quality, and cultural values. They can be analyzed at the global scale or for specific landscapes and seascapes.

Challenges: A key challenge is that changes in ecosystem services generally have strong correlations. That is, changes that cause increases in one group of ecosystem services often cause decreases in another group of ecosystem services. These tradeoffs among bundles of ecosystem services are not well understood. In management, they lead to unintended adverse consequences. These consequences often take systems across thresholds, degrade resilience, and impair the capacity of the system to respond adaptively to future environmental changes. Thus understanding the tradeoffs has fundamental importance for sustainable management.

Obstacles: In order to address this question, new frameworks for interdisciplinary collaboration are needed. Also there are significant needs for conceptual development, theoretical research, monitoring at global and regional scales, and empirical research at global and regional scales.

How will permafrost affect and be affected by global environmental change?

Permafrost is defined as ground that remains at or below 0°C for at least two consecutive years. Permafrost underlies approximately 25 % of the land area in the northern hemisphere and can be up to 1500 m thick. Under current climate-change scenarios, permafrost degrades from both the top and bottom, increasing the depth of the “active layer”, and the extent of talik formation.

The deepening of the active layer could trigger the massive decomposition of organic matter stored in the first three meters below surface. The most recent estimates put the organic carbon pool in permafrost at 50% of the global soil organic carbon pool. This pool is equivalent to twice the amount of carbon in the atmosphere. The decomposition processes would lead to the emission of vast quantities of greenhouse gases, including methane and carbon dioxide, which could greatly affect the global climate.

Under the sea, permafrost occurs as subsea permafrost. Its presence on Arctic shelves is intrinsically linked to the occurrence of gas hydrates which are released to the atmosphere through “holes” in the permafrost, called gas seeps. Its exact distribution on the shelves of the Arctic has not yet been correctly assessed, which hampers the attempts to correctly depict the mechanisms of gas hydrate occurrence and release.

In alpine areas, permafrost is responsible for the occurrence and the preservation of landforms that could evolve dramatically, resulting in large scale natural hazards for alpine valley settlements. In the Arctic, rapid coastal erosion of permafrost is expected to increase dramatically following the drastic reduction of summer sea ice extent, threatening the existence of Inuit communities.

Permafrost observation and monitoring is probably one of the most important challenges of the twenty-first century

More information on theIPA website

What is the role of land-use change for the present, past, and future evolution of the Earth?

The elements for possible consideration in this question have global dimensions. These elements range from carbon storage, food production, the water cycle, climate (including albedo), human societies, to migration. Answering this question requires full Earth system models that are not yet up to the task, in part because the processes that connect key elements of these models are not well constrained or understood. There are many causes of these deficiencies, including the fact that observing land-use change is difficult on the time and space scales needed for documenting and understanding key processes.

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!

How and why did genuine global changes happen? What were their local and global consequences on the physical environment, the ecosystems and the societies? What thresholds are involved?

Without paleoclimatic information, we would not know that atmospheric CO2 can vary naturally by up to 100 ppm on glacial-interglacial times, that abrupt climatic changes did occur on annual or decadal scales, that ice sheets may disrupt very rapidly, and basically that climate can change at all. These testimonies of how our Earth system is functioning are invaluable, yet still quite sparse and often not so well understood. They will likely deliver numerous further surprises. A great variety of climatic changes occured in the past, with many different amplitudes or consequences, and on many different time scales. When exceeding some thresholds, they were able to induce changes on the environment of past ecosystems and societies. These events should be traced back and quantified, before we can claim that we are in a position to predict future changes and their impacts.

How long can the Earth System sustain the present rate of human-induced global-environmental change?

Humans are modifying the planet at an alarming rate. Cropland and pasture now cover almost 50% of the entire land surface. This has led to massive habitat destruction, fragmentation and pollution and, together with overhunting, is causing a critical loss in biodiversity. Agricultural pollution is also having a devastating impact on aquatic and marine ecosystems which, together with industrial fishing, is causing collapse of key species populations within these ecosystems. Industrial pollution and burning hydrocarbons is causing polar warming which threatens to destabilize the remaining ice-sheets and reservoirs of methane stored in the polar oceans and permafrost. With populations in the US, China and India still rising, these clearly unsustainable practices are set to continue. The critical question is how long can planetary environmental processes continue to function before these human-induced changes trigger negative feedbacks that result in a switch to an alternate and less supportive Earth System state?

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.