Arctic meltdown not caused by nature
Rapid loss of Arctic sea ice - 80 per cent has disappeared since 1980 - is not caused by natural cycles such as changes in the Earth's orbit around the Sun, says Dr Karl.
The situation is getting rather messy with regard to the ice melting in the Arctic. Now the volume of the ice varies throughout the year, rising to its peak after midwinter, and falling to its minimum after midsummer, usually in the month of September.
Over most of the last 1,400 years, the volume of ice remaining each September has stayed pretty constant. But since 1980, we have lost 80 per cent of that ice.
Now one thing to appreciate is that over the last 4.7 billion years, there have been many natural cycles in the climate — both heating and cooling. What's happening today in the Arctic is not a cycle caused by nature, but something that we humans did by burning fossil fuels and dumping slightly over one trillion tonnes of carbon into the atmosphere over the last century.
So what are these natural cycles? There are many many of them, but let's just look at the Milankovitch cycles. These cycles relate to the Earth and its orbit around the Sun. There are three main Milankovitch cycles. They each affect how much solar radiation lands on the Earth, and whether it lands on ice, land or water, and when it lands.
The first Milankovitch cycle is that the orbit of the Earth changes from mostly circular to slightly elliptical. It does this on a predominantly 100,000-year cycle. When the Earth is close to the Sun it receives more heat energy, and when it is further away it gets less. At the moment the orbit of the Earth is about halfway between "nearly circular" and "slightly elliptical". So the change in the distance to the Sun in each calendar year is currently about 5.1 million kilometres, which translates to about 6.8 per cent difference in incoming solar radiation. But when the orbit of the Earth is at its most elliptical, there will be a 23 per cent difference in how much solar radiation lands on the Earth.
The second Milankovitch cycle affecting the solar radiation landing on our planet is the tilt of the north-south spin axis compared to the plane of the orbit of the Earth around the Sun. This tilt rocks gently between 22.1 degrees and 24.5 degrees from the vertical. This cycle has a period of about 41,000 years. At the moment we are roughly halfway in the middle — we're about 23.44 degrees from the vertical and heading down to 22.1 degrees. As we head to the minimum around the year 11,800, the trend is that the summers in each hemisphere will get less solar radiation, while the winters will get more, and there will be a slight overall cooling.
The third Milankovitch cycle that affects how much solar radiation lands on our planet is a little more tricky to understand. It's called 'precession'. As our Earth orbits the Sun, the north-south spin axis does more than just rock gently between 22.1 degrees and 24.5 degrees. It also — very slowly, just like a giant spinning top — sweeps out a complete 360 degrees circle, and it takes about 26,000 years to do this. So on January 4, when the Earth is at its closest to the Sun, it's the South Pole (yep, the Antarctic) that points towards the Sun.
So at the moment, everything else being equal, it's the southern hemisphere that has a warmer summer because it's getting more solar radiation, but six months later it will have a colder winter. And correspondingly, the northern hemisphere will have a warmer winter and a cooler summer.
But of course, "everything else" is not equal. There's more land in the northern hemisphere but more ocean in a southern hemisphere. The Arctic is ice that is floating on water and surrounded by land. The Antarctic is the opposite — ice that is sitting on land and surrounded by water. You begin to see how complicated it all is.
We have had, in this current cycle, repeated ice ages on Earth over the last three-million years. During an ice age, the ice can be three kilometres thick and cover practically all of Canada. It can spread through most of Siberia and Europe and reach almost to where London is today. Of course, the water to make this ice comes out of the ocean, and so in the past, the ocean level has dropped by some 125 metres.
From three million years ago to one million years ago, the ice advanced and retreated on a 41,000-year cycle. But from one million years ago until the present, the ice has advanced and retreated on a 100,000-year cycle.
What we are seeing in the Arctic today — the 80 per cent loss in the volume of the ice since 1980 — is an amazingly huge change in an amazingly short period of time. But it seems as though the rate of climate change is accelerating, and I'll talk more about that, next time …
Published 27 November 2012
© 2013 Karl S. Kruszelnicki Pty Ltd