Growing up it was common here in Texas to hear someone say “It’s gonna be a hot summer!.” It was a set up for sarcasm of all ages. This is Texas! It’s blazing hot every summer. But there is some relief afforded by the mild eccentricity of the earth’s orbit today:
Space.com — With such punishing heat as this, it may be shocking to hear that today at 11 a.m. EDT (1500 GMT), the Earth reached the point in its orbit where it is farthest from the sun in space. During aphelion, as it is called, the Earth will be 94,508,959 miles (152,097,426 kilometers) from the sun, or 3,106,399 miles (4,999,264 km) farther as compared to when the Earth was closest point to the sun (called perihelion) last New Year’s Day.
The difference in distance is 3.287 percent, which makes a difference in radiant heat received by Earth of nearly 7 percent.
Seven percent may not sound like much. But NASA’s Chief climate scientist, Dr James Hanson, explained to me once that just a few single watts above or below the average solar constant, about 1.366 kW/m averaged over the entire globe, spelled the difference between ice ages and warming trends in our past. A sustained drop of seven whole percent, if it lasted for decades, would probably send us careening into a new ice age.
Even changes in which hemisphere of the earth was tilted most toward or away to the sun during periods of closest and farthest approach played a big role on the advance and retreat of glaciers during the Pleistocene.
And you northern hemispherers have your summer around aphelion when our daytime star is at its furthest away* – how do you think we feel when we have our Aussie summer under tehperihelion sun?
Answer – Hot. Very hot indeed. Last summer was dubbed the angry summer here and broke well over a hundred records for extreme weather nationally. See :
http://climatecommission.gov.au/report/the-angry-summer/
* Like right now see :
http://www.slate.com/blogs/bad_astronomy/2013/07/05/aphelion_2013_the_earth_is_at_its_farthest_point_from_the_sun.html )
I think we have enough CO2 (and other greenhouse gasses) in the atmosphere to obviate the orbital geometry effects that make ice ages more or less likely to occur. If we don’t yet, we are getting close.
Putting this another way, a long term look at the climate “squiggles” (the things paleoclimate scientists put on graphs representing various measurements) from this point forward will simply look different than the way the squiggles have looked over the last ca 3.0 million years.