The Changing Water Cycle

As I spent this week thinking about what to write for this blog on water and environmental change, I started to realise just how much the two topics can cover. I mentioned this in my previous post but have started gaining a better appreciation for what that really means. I'm therefore going to pick out some themes I find interesting and write each blog post on such a theme, hopefully linking them with other blog posts and other blogs as I go. So for this blog, my theme is going to be the changing water cycle.

The environment is always changing; that much scientists know to be true. An integral part of the environment is the water cycle, and as the environment changes, so does the water cycle. I think it's important to say that the water cycle is changing due to changes in climate, but that the climate is also changing due to changes in the water cycle. It's a never-ending cycle and no one really knows where it began. So in this blog post, I'm just going to give a bit of context into why changes to the water cycle matter and what kind of changes can and have occurred.

Why does the water cycle matter?
Wikipedia defines the water cycle, or hydrologic cycle, as "the continuous movement of water on, above and below the the surface of the Earth". It also specifies that the water cycle involves exchanges of energy, which leads to temperature changes and therefore, influences climate. As you can imagine, this has implications for numerous areas of importance. Stocker and Raible (2005) stipulate that water has "profound effects on our planet's climate", from influencing the greenhouse effect through water vapour to ocean currents dictating regional climate patterns. This is reflected in the many processes that rely on water, not to mention the effects water has on human activity.

Put simply, natural processes rely on water and human processes rely on water. The water cycle controls the distribution of water, including where water is distributed, what state water is stored in (solid, liquid or gas), how often precipitation occurs, and how water moves around the world. For humans, understanding how the water cycle works is imperative for water management. For example, with changes to precipitation, water availability changes and mitigation techniques put into place to best use the water that is available. Arnell (1999) claims that precipitation will increase, but increase unevenly around the world, with large parts of land surface experiencing a reduction in precipitation. Using models, Arnell states that climate change will increase the number of countries under water resources stress by 2025. Clearly, changes to the water cycle matter to many different aspects of the natural and human environment.

What kind of changes have occurred?
The Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report working group report 'The Physical Science Basis' (2013) states that there is strong evidence for changes in the water cycle due to climate change. Observations since 1970s show increases in surface and lower atmospheric water vapour, shifts in ocean salinity due to the addition or removal of freshwater, changes in precipitation over land and an increased occurrence of extreme rainfall events. The report emphasises that ocean salinity is an effective way of measuring changes to the water cycle as it "naturally reflects and smoothes out the difference between water gain by the ocean from precipitation, and water lost by the ocean through evaporation". Durack et al. (2012) also use ocean salinity as a measure of the water cycle, and their study provides evidence that the water cycle has intensified since 1950. They also suggest that substantial intensification (16-24%) of the global water cycle will occur in a 2° to 3° warmer world.

I hope this brief introduction to the water cycle gives some context to what I'll be looking to blog about; including many more specific studies, with a focus on Africa. An area I'd like to explore further is anthropogenic effects on the water cycle and climate. The water cycle affects climate and climate in turn affects the water cycle. This is a mechanism as old as the Earth, and I'm interested in whether humans are negatively modifying this mechanism. For example, the proposed Anthropocene epoch (coined by Crutzen and Stoermer 2000) suggests that humans are preventing the natural glacial cycles of Earth through continued warming. The warming climate invariably has an impact on the water cycle; thus, humans could be seen as interfering with this natural occurrence.

Thanks for reading!