We’ve Been Through Climate Changes Before: But Mostly Cold Ones And Mostly In Our Far Distant Past

timelineby Dr. Sarah Green, reposted from Skeptical Science

“Yes, our climates change. They’ve been changing ever since the Earth was formed.” — Rick Perry

Previous major global climate changes were glacial cycles that happened long before human civilization developed.

The human species evolved during the last 2.5 million years. Our far distant ancestors survived through multiple gradual cycles of cold ice ages, but did not experience any previous “hot ages.”

We homo sapiens in our current form appeared only about 200,000 years ago. So our species has survived two ice ages. In each ice age global temperatures were colder by 4 °C. The warmest period ever experienced by early humans was [at most] about 1 °C warmer (global average) than today. That period occurred between the two most recent ice ages, 120,000 years ago (Eemian). Over the next 100,000 years temperatures gradually decreased into a new ice age. During that colder period humans began to expand out of Africa and across the globe. Ever since the Eemian much cooler temperatures have been the norm.

Human civilization is roughly 12,000 years old, as defined by the start of permanent settlements and agriculture. Agriculture became established as the glaciers retreated from the last ice age. Modern society has developed entirely in our current geological epoch, the Holocene. Global temperatures haven’t varied by more than ±1 °C since. There have been regional shifts in climate (Medieval Warm Period, Little Ice Age, etc), but since civilization began humans have never experienced a hotter global climate than now.

Going back further, over a million years or so, our pre-human predecessors experienced a series of long cold glacial cycles. Several short interglacial periods were as warm or slightly warmer than our current climate. For example, the climate 400 kyrs ago, was slightly warmer than now. But more typically for the last million years it’s been 4 to 8 °C colder. Each transition from warm to glacial ages and back took thousands of years, giving humans and prehumans many generations to adjust.

So, really, the climate hasn’t changed much since we settled into towns, invented plumbing, and started calling ourselves civilized.

Since humans and our human ancestors have been on Earth, average global temperatures have never been 3 °C warmer than now. In the next 100 years our children will be the first people ever to experience that kind of climate

But, perhaps Mr. Perry is thinking he’d like to live in a climate eons ago, closer to when the Earth was formed.

precambrian globe

Digging way back in time, we know that Earth’s climate has certainly been very different than it is now: 2 billion years ago there was not even any oxygen in the atmosphere. 550 million years ago high CO2 levels caused extreme greenhouse conditions. Humans were not around to care; the most advanced life form at that time was a flatworm. Humans could not physically survive over most of the planet in the age of the dinosaurs (Cretaceous, 100-65 Myr ago). Only very small mammals were beginning to evolve. Global average temperatures were 10-12 °C hotter than today. Most places on land were so hot that humans would risk fatal heat stroke every summer.

The geological record shows many ancient changes in climate, including massive ice ages, hot-house conditions, oxygen-free and acidic oceans, and massive extinction events. These changes happened millions of years before humans, most occurred before even primitive mammals, appeared on the scene. Previous climate changes were caused by orbital wobbles, solar fluctuations, and movement of continents. None of those effects are causing the current heating.

Sarah Green is the Chair for the Department of Chemistry at Michigan Technological University. This piece was originally published at Skeptical Science.

4 Responses to We’ve Been Through Climate Changes Before: But Mostly Cold Ones And Mostly In Our Far Distant Past

  1. Steve Bloom says:

    Hmm, a little too summary and gappy. It would have been helpful to mention the mid-Pliocene warm period (MWMP) experienced by australopithecines ~3.3 million years ago, with an emphasis on how equilibrium CO2 levels no higher than current resulted in much different conditions (albeit quite livable for us if you don’t count the nasty consequences of such a shift occurring on a scale of centuries, as now seems inevitable).

  2. dan allen says:

    Nice long-view perspective. The key question for the next 100+ years is this: Will agriculture still work in a destabilized climate?

    Probable answer from latest climate science: Probably not, especially in it’s current annuals-based form — and probably not at all in many key current food-growing regions.

    So what do we do? We need to (1) stop burning fossil fuels AND (2) we need to transition to an agriculture that stands a chance:

  3. Rabid Doomsayer says:

    Man in any form has not been through what we are going to go through. It is not the changes to date, it is the ones to come.

    Already agriculture has become much harder, weather chaos make it so hard to plan for crops that require fairly specific patterns of weather. Gentle rain a few weeks late can be devastating to a grain crop, never mind the drought, drought, flood pattern we seem to be in.

    Monoculture is only effective because of cheep inputs and regular climate. The cheep inputs, the good land will be in short supply. Polly cultures would appear to be the only answer going ahead. Polycultures get so much more per acre and are so much more resilient, but polycultures are more labor intensive.

    In past changes movment across vast distances has been the answer to survival. Our world political system does not favor mass migration.

    To survive we will need to be very flexible and yet flexibility is in short supply. Ingrained habit will not serve us so well in changing times.

    To use an old quote “You ain’t seen nuthin yet”

  4. Adam Sacks says:

    Contrary to Prof. Green’s assertions, we’ve known about abrupt climate change for a while. Her article warrants more careful review. See, for example, Richard Alley et al.,2003 Abrupt Climate Change.