By: Richard Allan
I was unfortunate enough to mildly injure my middle finger by typing too frenetically on a train journey from Toulouse returning from an Intergovernmental Panel on Climate Change meeting. I soon forgot about this by luckily stepping on a rusty nail the next day while demolishing a shed and following a tetanus booster I am back to assessing research and preparing text outlining our knowledge of how the water cycle is expected to evolve as the planet continues to heat up from the emissions of greenhouse gases.
Climate change will impact people and the ecosystems upon which we all depend through aspects of the water cycle. The physics of the atmosphere, oceans and land surface tell us that climate change will alter and in many cases intensify events that cause there to be too little usable water to meet our needs or produce too much water at once as deluges inundate drainage capacity. Thousands of person-years of work crams state-of-the-art scientific knowledge into millions of lines of computer code required to make realistic simulations of our climate. These are combined with observations of the real world and physical interpretation to assess the range of future possibilities for policy makers to plan effectively.
No one is killed by global average temperature yet understanding and monitoring how the Earth’s energy and water cycles are currently evolving is a challenge for our observing systems and a test of our basic understanding of the climate system. At the risk of further injuring my finger, I’ll get straight to a simple depiction of how our global climate is evolving in the diagram below. This shows departures from the usual monthly values in global average surface temperature, atmospheric moisture, precipitation and the energy accumulation driving climate change. These are based on surface measurements and satellite observations where gaps in coverage are filled with a meld of observations and simulations called “reanalyses”. The grey shading shows results from “CMIP6”, the latest generation climate simulations, here run in atmosphere-only “AMIP” mode (fed with the observed sea surface temperature and sea ice as well as realistic changes in radiative forcing agents that are perturbing our climate) that are directly comparable to the observations.
Figure 1:– Simulations and observations of global average temperature, moisture, precipitation and heating balance between absorbed sunlight and emission of infrared radiative energy to space (extended from Allan et al. 2014a,b).
The ocean temperature has been increasing around 0.2oC every decade, primarily due to rising atmospheric carbon dioxide concentrations. This trend is punctuated by natural climate fluctuations. For example the 1991 eruption of Mt Pinatubo in the Philippines cooled the global climate for a few years as ejected particles reflected sunlight back to space (seen by the dip in Earth’s heating rate) while slow, random sloshing about of the ocean briefly warms climate in El Niño events (as marked on the diagram in 1998 and 2016). The temporary warmth is eventually lost to space as seen by the dip in Earth’s heating rate as El Niño takes hold.
As the planet has warmed, both satellite estimates and surface observations show that moisture in the atmospheric column becomes more plentiful (a 6-7% increase for each oC of global warming). This is expected from basic physics and simulations of the atmosphere reliably recreate the real world. This increases our confidence in the most powerful amplifying effect on climate change, the water vapour feedback in which warmer air with more moisture traps more radiative heat. A greater abundance of moisture also drives an intensification of the water cycle with greater flows of moisture from regions of strong evaporation into storms. This is intensifying rainfall events and the severity of flooding where heavy rainfall occurs. This is also seen in warm El Niño events with a peak in precipitation globally, although the impacts are felt more by the redistribution of rainfall and unusual weather patterns.
The global precipitation rate is a slave to Earth’s energy balance rather than moisture which is why only small changes in global precipitation (a 1 or 2% increase for each oC of warming) are expected in the short term as seen in the simulations and satellite data. Satellites and ocean measurements monitoring Earth’s energy balance and although this fluctuates from year to year there is a continual accumulation that is heating the planet equivalent to every person currently alive on Earth each using twenty-two 2-kilowatt electric kettles to boil the ocean (babies would probably need supervision).
Current indicators of climate change are vital in strengthening understanding of how the climate is changing currently and will change in the future and what is needed to avoid and adapt to associated damaging effects. Earth observation from satellites and other observations are vital in verifying, questioning and improving this understanding. And with that I’m off to the UK’s National Centre for Earth Observation annual conference to learn more!
Allan, R. P., C. Liu, N. G. Loeb, M. D. Palmer, M. Roberts, D. Smith and P.-L. Vidale (2014) Changes in global net radiative imbalance 1985-2012, Geophysical Research Letters, 41, 5588-5597, doi:10.1002/2014GL060962
Allan, R. P., C. Liu, M. Zahn, D. A. Lavers, E. Koukouvagias and A. Bodas-Salcedo (2014) Physically consistent responses of the global atmospheric hydrological cycle in models and observations, Surveys in Geophysics, 35, 533-552, doi:10.1007/s10712-012-9213-z