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.

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?

How sensitive is the oceanic conveyor belt to the influx of glacial melt water and/or decrease of seasonal ice formation in the Arctic Ocean/North Atlantic Ocean?

The impact of all other global warming issues may be minimized if there is a threshold of fresh water influx and/or decrease of seasonal ice formation which may soon be exceeded and thus dramatically slow or reverse the trend of global warming.

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 to deal with the uncertainties associated with Earth system research, especially policy-relevant areas?

In addition to climate change uncertainties, we are still lack of knowledge about interactions of land-atmosphere, Atmosphere-sea, aerosol-climate, chemistry-climate, and more importantly human-environment relationships.

What are the key regional drivers of future climate change?

Globally, greenhouse gas forcing is the key driver in policy-relevent climate change (ie. over the next 20, 50, 100 years). Regionally -at the scales people live, ecosystems function, water is obtained and crops grown, other forcings can dominate. Land cover change, urbanization, industrial aerosols etc can all have regional fingerprints that while globally small are locally dominant. Other modes of variability, ocean-atmopshere coupling, land-atmopshere coupling, orographic effects etc all can be locally dominant drivers even if they are lost in any global measure of climate change. A research program to understand drivers of climate change at the scales that people live is hugely challenging at a scientific level, technical level for the modelling and in terms of research at the interface of risk and vulnerability.

How will marine communities respond to the interactive effects of ocean temperature change, increasing CO2 levels, decreasing pH, and other human-induced forcing over the next century?

Oceanic autotrophs account for about half of total global primary production. The food webs they support are important for a wide variety of human endeavors involving the sea, ranging from fisheries to coastal protection. Yet little is known about the synergistic effects of ongoing global changes on marine communities. Early evidence suggest that some organisms are highly sensitive to pH changes while others are available to take advantage of higher levels of dissolved carbon dioxide. How will species and communities respond to the sum of all forcing agents and at what timescales?

The principle obstacles to making progress in developing answers to this question revolve around the expense and difficulty of conducting ecological, physiological, and biogeochemical/climatic studies at sea.

What are the key processes that control the size and shape of the global ocean overturning circulation, and how will they combine to influence its future evolution?

The meridional overturning circulation (MOC) of the global ocean has a governing role over the Earth’s climate, a consequence of the enormous capacity of the ocean to store and redistribute heat, freshwater, carbon dioxide and other climatically-important tracers. This results in the MOC being arguably the most prominent feature of the climate system that transcends physical, chemical and biological processes, linking atmosphere-ocean-ice interactions with the carbon cycle and other biogeochemical processes, including ocean acidification. The MOC is known to exhibit variability on timescales from months to millennia, and has the potential to induce abrupt change in climate, most pertinently in the region of the North Atlantic.

The mechanisms governing the strength, shape and variability of the overturning circulation are not fully understood, but include wind forcing, buoyancy forcing and mixing. It has been postulated that the polar regions, where global climate change impacts are predicted to occur earliest and with largest amplitude, play a disproportionately important role in these. A combination of theoretical studies, model simulations and observational analysis (including monitoring of the present state and assessment of paleo conditions) is essential to better understand these mechanisms and to predict future changes.

Issue of resilience of our Oceans vis-a-vis climate and geosphere/biosphere changes, as all sort of pressures (natural and anthropogenic) are affecting today the Oceans Realm? How much resilient our Oceans will be for the next coming decades?

Important issue as oceans constitute more than 2/3 of our planet.
Do we have good models of prediction to assist decison-makers in their planning of oceans management and governance?

How do different forms of pollution including increased nutrients and changes in the trophic dynamics of marine ecosystems affect the oceans ability to regulate climate?

Oceans are the key driver of climate change globally, yet pollution into marine and coastal waters continues as does over-exploitation – if we are to address all of these issues in an integrated way, then we need to know how these activities impact on climate change processes now and in the future.