Antarctica is already feeling the heat of climate change, with rapid melting and retreat of glaciers over recent decades.

Ice mass loss from Antarctica and Greenland contributes about 20% to the current rate of global sea level rise. This ice loss is projected to increase over the coming century.

A recent article on The Conversation raised the concept of “climate tipping points”: thresholds in the climate system that, once breached, lead to substantial and irreversible change.

Such a climate tipping point may occur as a result of the increasingly rapid decline of the Antarctic ice sheets, leading to a rapid rise in sea levels. But what is this threshold? And when will we reach it?

What does the tipping point look like?

The Antarctic ice sheet is a large mass of ice, up to 4 km thick in some places, and is grounded on bedrock. Ice generally flows from the interior of the continent towards the margins, speeding up as it goes.

Where the ice sheet meets the ocean, large sections of connected ice – ice shelves – begin to float. These eventually melt from the base or calve off as icebergs. The whole sheet is replenished by accumulating snowfall.

Floating ice shelves act like a cork in a wine bottle, slowing down the ice sheet as it flows towards the oceans. If ice shelves are removed from the system, the ice sheet will rapidly accelerate towards the ocean, bringing about further ice mass loss.

A tipping point occurs if too much of the ice shelf is lost. In some glaciers, this may spark irreversible retreat.

Where is the tipping point?

One way to identify a tipping point involves figuring out how much shelf ice Antarctica can lose, and from where, without changing the overall ice flow substantially.

A recent study found that 13.4% of Antarctic shelf ice – distributed regionally across the continent – does not play an active role in ice flow. But if this “safety band” were removed, it would result in significant acceleration of the ice sheet.

Antarctic ice shelves have been thinning at an overall rate of about 300 cubic km per year between 2003 and 2012 and are projected to thin even further over the 21st century. This thinning will move Antarctic ice shelves towards a tipping point, where irreversible collapse of the ice shelf and increase in sea levels may follow.

How do we predict when will it happen?

Some areas of West Antarctica may be already close to the tipping point. For example, ice shelves along the coast of the Amundsen and Bellingshausen Seas are the most rapidly thinning and have the smallest “safety bands” of all Antarctic ice shelves.

To predict when the “safety band” of ice might be lost, we need to project changes into the future. This requires better understanding of processes that remove ice from the ice sheet, such as melting at the base of ice shelves and iceberg calving.

Melting beneath ice shelves is the main source of Antarctic ice loss. It is driven by contact between warmer sea waters and the underside of ice shelves.

To figure out how much ice will be lost in the future requires knowledge of how quickly the oceans are warming, where these warmer waters will flow, and the role of the atmosphere in modulating these interactions. That’s a complex task that requires computer modelling.

Predicting how quickly ice shelves break up and form icebergs is less well understood and is currently one of the biggest uncertainties in future Antarctic mass loss. Much of the ice lost when icebergs calve occurs in the sporadic release of extremely large icebergs, which can be tens or even hundreds of kilometres across.

It is difficult to predict precisely when and how often large icebergs will break off. Models that can reproduce this behaviour are still being developed.

Scientists are actively researching these areas by developing models of ice sheets and oceans, as well as studying the processes that drive mass loss from Antarctica. These investigations need to combine long-term observations with models: model simulations can then be evaluated and improved, making the science stronger.

The link between ice sheets, oceans, sea ice and atmosphere is one of the least understood, but most important factors in Antarctica’s tipping point. Understanding it better will help us project how much sea levels will rise, and ultimately how we can adapt.

Felicity Graham, Ice Sheet Modeller, Antarctic Gateway Partnership, University of Tasmania; David Gwyther, Antarctic Coastal Ocean Modeller, University of Tasmania; Lenneke Jong, Cryosphere System Modeller, Antarctic Gateway Partnership & Antarctic Climate and Ecosystems CRC, University of Tasmania, and Sue Cook, Ice Shelf Glaciologist, Antarctic Climate and Ecosystems CRC, University of Tasmania

This article was originally published on The Conversation. Read the original article.

Many of us never think about who gets to use the drops of rain that fall from the sky. But it’s an increasingly pertinent question as more people look to collect rainwater as a way to conserve water, live off the grid or save money on water bills.

As a result, many states in the arid West are now asking whether rain barrels are allowed under existing law and policy and, in some cases, are setting limits on the practice of rainwater catchment.

Colorado has gone further than any of its neighbors by requiring a permit for any rainwater collection. Meanwhile, Utah put rainwater harvesting rules into effect in 2010 with some restrictions, and Washington legalized rainwater collection in 2009, while leaving the state the “ability to restrict if there are negative effects on instream values or existing water rights.”

Why this worry over rainwater harvesting?

If everyone captures the rain that falls on rooftops and through downspouts of homes, the argument goes, then the water will never reach the rivers and streams. If this happens, existing water users may not be able to access their rights to use the water.

This concern, however, overstates the issue and risks missing more concrete opportunities for water conservation and efficiency. A more effective way to address decreasing water supply would be for states to apply the legal principles prohibiting waste and demanding reasonable water use, which have long been embedded in any right to use water.

U.S. water law, east and west

Both the rainwater collectors and the existing water rights holders, such as irrigators or municipalities with water rights to river flows or groundwater sources, believe they have a fully private interest in any water they use.

Throughout the United States, however, the law recognizes the public nature of water. Under the public trust doctrine, each state holds title to the water within the state in trust for the people of the state.

Given the competing demands for water use, principles of U.S. law seek to balance these competing needs and uses to ensure that the public’s rights to water are protected.

In the eastern United States, there is the riparian system that protects reasonable use of water among all landowners along rivers or streams. In the western part of the country, the doctrine of prior appropriation requires a permit to use water based on showing that the water will be put to beneficial use without waste.

The public nature of water ensures that individual private interests never fully control who gets access to water and when, where and how water is used. In fact, when an individual has a right to use water, that right is known as a “usufructory” interest – that is, the right to use the water without owning the water itself.

Granting a usufructory interest – something that doesn’t fully privatize a water right – makes good sense when you think about the nature of water.

Short of putting water in a bottle and selling it by the ounce, water is difficult to possess and reduce to ownership. It is a shared resource that is used over and over again as the molecules of water make their way through the hydrologic system.

Water falls from the sky, runs along the ground and percolates into the groundwater system. It is taken up by plants and trees, consumed by people and animals, and eventually makes its way through one mechanism or another, back into the groundwater or surface water sources, only to flow further down the system to be used again or eventually evaporate back into the atmosphere to start the process all over again.

Private ownership of drops of water presents a complex problem not only as a legal matter, but as an ethical public policy choice as well.

The debate over rainwater collection demonstrates this complexity.

Don’t homeowners in Colorado have the right to collect rain that falls on their rooftop? At the same time, doesn’t a senior water right holder have a right to have the rain enter the stream so that their right can be satisfied?

Our legal system evolved ways to deal with this complex reality, with our state governments empowered to manage this resource among competing interests on behalf of all of us.

In the eastern United States where rainfall is plentiful and competing uses for water are rare, the riparian system allows any landowner adjacent to a water source to use its water. If there is a conflict about the quantity of water available for a certain use, that conflict is resolved by using legal standards to sort through the reasonableness of each individual’s use.

In the western United States, where competition among users has always been more commonplace, each individual state requires a permit for water use. These permits are awarded pursuant to the doctrine of prior appropriation. For example, irrigated agriculture often holds senior water rights (issued under state law) and Indian tribes often hold even more senior rights (based on federal law).

When conflict arises, disputes are resolved using the legal principle of first-in-time, first-in-right that protects the most senior, beneficial, nonwasteful uses of water. Or at least that is the theory.

Water waste and powerful interests

So how does this relate to the regulation of rainwater harvesting?

If the primary concern is that somehow rainwater barrels will limit the amount of water in the system, reduce availability of water and potentially impact existing rights, then there may be better ways to address this concern.

Rather than devoting resources to regulating individual rain barrels – a logistically difficult task – it may make more sense for state water agencies to get serious about enforcing principles of waste.

To enforce waste reduction policies, water resource management agencies in each state would need to set standards on how much water is needed to carry out a particular use. They then would need to measure water use to ensure that individual permit holders are not taking more water than what is necessary to accomplish their purpose.

Many longstanding water users take more water than they need, under the principle of use-it-or-lose-it. In western water law, if you don’t use the water, you risk forfeiting your water right. As a result, many users divert the full quantity of their water right whether that amount is needed or not.

If the states crack down on waste, it will bring this longstanding practice into the spotlight. Existing water users may be faced with calls to increase efficiency and to decrease the rate of diversion.

For decades, there has been a persistent reluctance to address waste because it would involve scrutinizing water use practices among some of the most powerful interests in the state.

But by addressing the thorny problem of waste, state agencies could make more headway in securing reliable water supplies and certainly could have a more significant impact on water supply than regulating rainwater catchment.

In the end, we may face tough public policy choices about whether and how to regulate rainwater catchment. But before we go in this direction, policymakers should take a careful look at whether existing larger-scale water users are complying with longstanding principles of nonwaste and reasonableness embedded in U.S. water law.

Adell Amos, Associate Dean for Academic Affairs, Associate Professor of Environmental and Natural Resources Law, University of Oregon

This article was originally published on The Conversation. Read the original article.