From atmospheric changes, to timelapse imagery from Google Earth...our planetary presence is hard to miss.

This past week has seen the concentration of carbon dioxide (CO2) in Earth's atmosphere reach a level of 400 parts-per-million, a value the planet hasn't seen since several million years ago. To put this into some kind of context let's take a look at the variation in CO2 over the past half century or so - via the classic 'Keeling Curve'.

Atmospheric CO2 concentration in parts per million as a function of time (Scripps/NOAA).












There are two basic features of this plot. First is that it wiggles up and down on an annual basis. That's because the Earth has seasons and oceans and landmasses with living organisms that absorb and generate CO2. Plants, for example, tend to absorb CO2 in summers when they're growing, and can release CO2 in winters when they're rotting or dormant. The planet is also lopsided in the sense that there is more 'habitable' continental land mass in the north than in the south - otherwise the net contribution to CO2 variation between northern summer and southern winter, or vice versa, would cancel out.

The second feature of this plot is that CO2 concentration is increasing with time. Why? Well, it's simple, it's because of us. This is seen most starkly if we take a look at a rather longer timeline - made using ice-core measurements of atmospheric CO2 (since our ancestors weren't monitoring the atmosphere for us). It begins going uphill just around 1760 - the start of the Industrial Revolution.

CO2 concentration back to the 1700s - it starts rising around 1760 (Scripps)

You can begin to see just how steep the rate of change has become since the mid 20th century if we go even further back and look at the past 800,000 years.

See that little spike at the right hand side up to 400 ppm? That's us, today. Although CO2 concentrations have been far from stable over the past 800,00 years, they take a sharp upward turn right in line with the rise of industrialized human civilization.

CO2 atmospheric concentrations from ice core sampling - for past 800,000 years (Scripps).

Now of course the ice core data are not perfect, there could potentially be some other spikes in there that get washed out in the measurements, but we would probably spot anything like our present fast rise.

How far back do we need to look to hit similar CO2 levels to today's? It's not quite clear, but it might be around 3 to 4 million years. A study by Bartoli, Honisch, and Zeebe in 2011 measured boron isotopes in the mineral shells formed by types of plankton, and suggests this was the case.

Looking further back, across more of the immense history of the planet, and we see that CO2 wanders all over the place. For example, here are some estimates showing both measurements and computer models (timeline flipped from previous plots).

500 million years of CO2 concentration on Earth (This figure was prepared by Robert A. Rohde from published data and is incorporated into the Global Warming Art project).











There's a lot of uncertainty, but it's clear that even our present 400 ppm CO2 concentration is small compared to what it has been tens to hundreds of millions of years in the past. Amounts that were five to twenty times higher than today seem more like the norm.

But humans, and the world we find around us, didn't exist back then. These were periods where the Earth, our lovely home planet, would have felt about as alien as some of the exoplanets we're now discovering in the surrounding universe. The environmental chemistry, the fauna (not always any flora), and the climate may have never before been a match for what it has been the past million years or so.

Like it or not, the historically low trough in the CO2 concentration of yesterday is a defining characteristic of the window of opportunity where our peculiar ape-related ancestors managed to get a foothold. The fascinating but rather terrifying thing is that we've now gone global, and we've learned how to extract vast amounts of energy from our environment, driven by an extraordinary ability to innovate and survive. By doing so we've altered that window, significantly changing the chemical composition of the atmosphere. And although I'm not going to discuss it in detail here, simple physics tells us what's going to happen next. You cannot deny basic thermodynamics.

This is merely one change our presence has brought to the planet. There are many others. In fact you can now be a first-hand witness to the visible alterations going on around the globe through the magic of Google Earth. Terabytes of imaging data from 1984 to the present have been stitched together to allow a timelapse view of just about any part of the world. Go check it out here.

Even in this day and age of planetary awareness it's pretty amazing to see just how infested a place it is. Here's an example, Las Vegas, 1984 to 2012.

What happens in Vegas apparently spreads from Vegas....

Once you're done with looking at other examples, try entering your own's quite educational.

Is it all doom and gloom? Yes and no. Clearly we're testing the limits, we have a good chance of pushing our planet (if we haven't done so already) to a place it hasn't been to for millions of years - the kind of place that we might not like. The kind of place that might kill us. But we're also amazingly clever, or else we wouldn't know that we're doing this. So what's going to happen?

I wish I knew. But as a scientist, and an optimist, I can't help but notice that if we're serious about looking for life beyond the Earth and if we're serious about looking for complex, technological life, it's this kind of filthy disregard for planetary equilibrium that we should be sniffing for. We might have to wait a while if we find suitable candidate worlds to aim our telescopes at, but it's conceivable we might catch some other life-form making precisely the same mistakes we are.

Through early morning fog I see

Visions of the things to be

The pains that are withheld for me

I realize and I can see

That suicide is painless

It brings on many changes

And I can take or leave it if I please

(Johnny Mandel and Michael Altman, 'Suicide Is Painless'/M.A.S.H. theme)