Climate change and climate variability

​At the Marlborough Earth Day party, held at A & P showgrounds last April, a number of visitors were asked if there were aspects of climate change that they didn’t understand.  Shannon asked, “Why does global warming lead to more climate variability?”
This is a good question because it isn’t immediately apparent why simple warming of the planet should lead to greater variability in our weather.  Global warming – the gradual warming of our atmosphere due to build-up of heat trapping gases – is having both direct and subtle effects on short-term weather events and the climate over relatively long periods of time.  The direct effects are easy to visualise; a warmer atmosphere leads, on average, to a warmer climate worldwide.  At high latitudes – the Arctic and Antarctic – this has resulted in widespread melting of land and sea ice.  At low latitudes, near the equator, it has led to some extreme heat waves, like those recently experienced in India and Pakistan.  More subtle effects include global warming’s contribution to water evaporation and to changes in both air and sea currents.
The impact of global warming on water evaporation lead both to more intense rainfall and more severe droughts.  Warm air can hold more moisture than cool air, so the evaporation of water from both land and sea is enhanced by warmer air temperatures.  And since what goes up must come down, more moisture in the atmosphere results in more rainfall.  For example, the extensive flooding in the south central US as a result of Hurricane Harvey last year can be related to rising sea surface temperatures in the Caribbean Sea, feeding more moisture to the storm than otherwise.  Conversely, in places where it doesn’t rain, warmer air temperatures result in water evaporating from land more readily, resulting in more severe drought.  The hotter it gets, the faster soil dries out.
Less apparent is how the very cold winter storms experienced by Europe and North America last winter can be related to global warming.  Although research is ongoing, it appears to be due to a weakening of the northern jet stream – a river of air moving around the planet at the mid-latitudes.  In the winter months, this brings a succession of Pacific storms to the west coast of North America.  The jet stream also surrounds and bottles up cold air in the Arctic (the polar vortex), preventing it from breaking out into the mid-latitudes.  It seems that as our climate has warmed, the jet stream has weakened, allowing both cold Arctic air to escape south and relatively warm southern air to stream north into the Arctic.  While Europe and North America were freezing under unusually cold weather last winter, some areas of the Arctic experienced unusually warm winter weather.  Temperatures in parts of the Arctic averaged 20ºC warmer than usual – the warmest Arctic winter temperatures on record.
In the long term, global warming could have an even more dramatic effect on climate.  Scientists have noticed a weakening in a portion of the Gulf Stream that heads into the North Atlantic Ocean and this weakening appears to be related to Arctic warming.  The Gulf Stream is a clockwise current of warm tropical water in the mid-Atlantic Ocean between Europe and North America.  A portion of this current flows into the North Atlantic, keeping Northern Europe warmer than it would otherwise be, based upon its latitude on the planet.  For example, the UK, while at the same latitude as Labrador and southern Alaska, does not share their far colder climates.  It is this northern portion of the Gulf Stream that keeps the UK and the rest of Northern Europe warmer than they would be otherwise, so a weakening of this current would result in a cooler climate for Northern Europe – the opposite to what we might expect with global warming.