In this lecture we provide scientific evidence why climate change is a planetary boundary, and the basis for defining the boundary for climate change.

It originates, not surprisingly, from the fantastic scientific explorations of our recent paleoclimatic conditions on Earth, again the stable climatic state we’ve had in the Holocene, shown here over the past 2000 years of reconstruction of temperature which varies at a maximum of +/- 1 degree or 2 degrees Celsius.

And as you see at the end the extraordinarily rapid pace of temperature rise in the world over just the past 150 years, since the Industrial Revolution, and our initial large scale emission of greenhouse gases from our industrial development.

This is the basis for the fundamental evidence that builds up the arguments around climate change. It becomes even more dramatic if you connect the past with the future, which is shown in the next graph showing the IPCC projections up until the end of this century.

And it’s absolutely extraordinary to see the Holocene stability which is again this churning up and down with a +/- 1 degree Celsius, and the fact that we are today heading on average along a pathway that will take us to in the order of 4 degrees Celsius warming during this century. And I think it’s absolutely clear just from this graph that we are at risk of pushing ourselves very rapidly outside of the Holocene stability.

Now up until today we have already increased global average temperature levels within the order of 0.8-0.9 degrees Celsius over the past 100 years.

What you see here is the distribution of that heat across the planet based on modeling and observations, and what you see is that in fact many regions in the northern hemisphere have actually even more warming already today, which is for example affecting one of the regulating systems, namely the polar regions in terms of its feedbacks in the stability of the Earth system.

The next insight building up the evidence for a climate boundary is that sea level rise is occurring at the pace of the projections we have or even faster. In fact much of the evidence today points at the risk of us underestimating the pace of sea level rise, particularly because we’ve underestimated the rate of melting which potentially could be irreversible in parts of Antarctica.

We’re also seeing unfortunately increasing robust analysis looking into the future that we’re following what I would actually call a disastrous pathway that leads us on average to three, four, potentially even higher, warming in this century. And that is coming out of the Intergovernmental Panel on Climate Change (IPCC) last, the most recent 5th assessment.

And here you see the synthesis graph showing the different scenarios to the future. The red line is the pathway that takes the world towards a totally undesired not Holocene-like state of 4 degrees C warming, and we know unfortunately that we’re following this path. So this is again increasingly showing that we need to do something very rapidly in putting a boundary on climate change to avoid moving outside of a desired state.

The next piece of analysis is related to the risk of tipping points and risk. And here science is advancing in a very profound way. We’re understanding the climate system much, much more in detail, and particularly how the climate system interacts with the other planetary boundary processes such as land, water, oceans, and biodiversity.

And this graph may seem a little bit complex, but it’s a really important insight that is coming out of the three last IPCC assessments. So what you see here is a risk assessment shown in red ambers, which has become a seminal and very famous set of graphs, of the risk analysis of the 3rd assessment to the left, the 4th assessment in the middle, and the most recent 5th assessment to the right. And what I want you to look particularly to is the column furthest to the right in each assessment which has a small darkened little block attached to it, which is the assessed risk from science of large scale discontinuities.

To put it in simple language, the risk of human-induced catastrophic tipping points. So this is the risk of us destabilizing the entire monsoon system, or irreversibly melting the Greenland ice sheet. And if you look carefully in the third assessment the risk of such large-scale discontinuities was assessed to occur at a point where the warming reached in the order of 4 degrees C. So on the Y-axis you have average temperature at which we risk these kind of catastrophic tipping points. But as knowledge advances, as our understanding of [how] the complex Earth has evolved, just a few years later in 2007 with the fourth assessment, as you’d see if you look carefully the threshold at which catastrophic tipping points can occur is down in the range of 2-3 degrees C.

And now in the most recent 2013 fifth assessment you see that the assessment from science is that these kind of large-scale discontinuities could actually occur even lower – in the order of 2 degrees C warming. And the reason why this is occurring is that we’re understanding more and more about resilience, about the risks that we have surprise and thresholds in the Earth system. And this to me is the most fundamental piece of evidence showing that a planetary boundary approach on climate is absolutely necessary because for one, at already very low temperature rises we today have evidence enough to say that the likelihood of large scale catastrophic changes is highly probable. And secondly, it’s highly uncertain. It’s so complex that we need to apply a precautionary principle where a boundary position is a position of safety beyond which we enter this area of uncertainty. And just to really hammer that point home, when you translate the latest assessment of the IPCC, our 5th assessment in terms of risk, something absolutely astonishing falls out.

We are today, in 2014, at a concentration of greenhouse gases for all gases, so carbon dioxide plus the other gases including methane, nitrous oxide, chlorofluorocarbons, and the short-lived climate forcers, including soot and sulfates and organic carbon, we are at 450 ppm.

Now we have taken the data in the IPCC and just translated them in probability of reaching different degrees at 450 ppm, and that is shown in this graph.

So on the X-axis you have temperatures and on the Y-axis you have the probability of reaching that degree of temperature at 450 ppm, at our current concentration of greenhouse gases. And look at the point which I’ve put on this graph which is 6 degrees C. I’ve taken an extreme warming, 6 degrees C is something totally outside of anything we can imagine, it’s an uninhabitable planet, it’s a degree of warming which any person, even a climate skeptic, would agree is totally unacceptable for humanity.

What’s the probability at 450 ppm according to the latest IPCC that we reach 6 degrees C Well on the Y-axis you see that the probability is a staggering 1.6%. Now what does a percentage, a probability of 1.6% mean? Well to give you and equivalent it would be the same as accepting that we have 1,500 aircrafts crashing every day. So it’s a probability level for catastrophic events, which in in any other sector of society would never, ever accept.

In fact, some of  the large reinsurance companies after the IPCC released its report, clearly pointed out that we’re reaching a point of risk which goes beyond the point where they potentially can no longer issue, insurances because they can not be liable for the large scale costs that would be incurred if these kind of catastrophic events would be allowed to happen. So we’re entering truly a danger zone with regards to climate.

This is shown clearly in the next slide here on our analysis of how much forcing we are loading on the climate system. So what you see here is the last half million years how we are able to reconstruct in a very, very adequate way how much greenhouse gases we have in the atmosphere, how much forcing that includes. Forcing is the amount of watts, the energy that is trapped per square meter because of the greenhouse gases in the atmosphere. And look at the future.

What you see here is how we are rapidly moving out of the Holocene, moving out into a forcing and temperature rise which is way, way outside of the Holocene equilibrium. Now when we all take all this science together and synthesize it to define the boundary, we then apply our theory of a safe operating space, an uncertainty zone, and a danger zone, and we find that the science indicates that at the range of between 350 ppm and 450 ppm the science is well in agreement that here we have a risk of crossing catastrophic thresholds. And therefore we apply the boundary at the safe lower end of that uncertainty which is 350 ppm for carbon dioxide. And there you have it, that’s the way we place the boundary for climate change.