Now we have an idea of how we can measure past temperature and past concentration of CO_2. Let's look at what these indirect measurement of proxies tell us about temperature in the relatively recent past, going back to the exit from the last ice age and going back a lot long more time. Let's begin from the recent past, the past 2,000 years. Here we see these are obtained from so-called Greenland proxies. We see in the last part of the graph to the right there is a sharp increase in the temperature anomaly. If you notice in the top right hand of the graph, there is a black line super-imposed on the blue line, that is very important. The blue line is the temperature reconstruction obtained from the ice core proxies in Greenland, and the black line is the direct observation of a temperature. The very good news is that the black line almost exactly superimposes the blue line. This gives us confidence that with technique that we have discussed is actually good at determining past temperature, very important. This is the past 2,000 years, 2,000 years sounds like a lot, but it's actually just nothing in even biological times. Let's go back 12,000 years. Here we are observing several important things. Starting at the far left, we have the exit from the last ice age. The temperature was increasing a lot. At the very right of the graph, you see there is still that black little direct measurement, but now it is a tiny fraction of a temperature records that we have, everything else is reconstruction with proxies. As you can see, the temperature increase now is not bringing us above temperature ranges that we have encountered about 6,000 or 8,000 years ago. However, the human civilization has recurrently know was not around, of course we were around as a biological species, but in historical terms, it was very different condition 6,000, 8,000 years ago. Now, this goes back a long, long period of time. Here, we're going back 800,000 years. We have temperature reconstruction over these very extended period. We see obvious periodicities that we will understand later on when we do the Milankovitch cycles. But again, we have to put everything in perspective. Look at the far right, which is where we are now, because the axis here, runs right to left, and it is thousands of years before the present, and we see the current climate situation, the current committee event compared to what we have seen in the past. Now, 800,000 years sounds like a lot but what can we say if we look back even further? Now we're really going into the geological past of the Earth. This is very important to understand the distinction between Greenhouse Earth and Icehouse Earth. Over the last 2.5 billion years, there have been five big ice ages of durations from tens to hundreds of millions of years. For the rest of the period, the majority of the period, the Earth appears to have been in what is called a greenhouse state, which means largely ice-free at the poles. During the greenhouse period, there are no permanent ice caps at the poles. Icehouse periods should not be confused with glacial and interglacial period. Interglacial periods occur during an icehouse period and tend to last less than one million years. They are linked to what we will see at the Milankovitch cycles, we'll understand what they are later on. A greenhouse period is a period in which there are no continental glaciers, the level of CO_2 is high, and the sea surface temperature ranges from 28 centigrade in the tropics to just about zero in the polar regions. Greenhouse and this is very important, should not be confused with hothouse. Hothouse Earth corresponds to an irreversible tipping point with a runaway greenhouse effect, similar to what we believe has happened on Venus. The Intergovernmental Panel on Climate Change, which is a very reliable source of information on climate change matters, states that, "A runaway greenhouse effect, analogous to what we have encountered, or we think has happened on Venus, appears to have virtually no chance of being introduced by anthropogenic activities." There is a bit of good news. Let's go back to greenhouse and icehouse. During a greenhouse phase, the Earth conditions are very favorable to life. Dinosaurs appeared during greenhouse period, and during the same greenhouse phase, land animals began to cover the continents. Water now, the Arctic seas, where field, one of field is an exaggerated, there were lots of reptiles swimming in what are now the Arctic Seas. Birds, mammals, and plants evolved. The important point is that the earth has been in a greenhouse state for about 85 percent of its history. But we're not in a greenhouse phase now. So we are looking at global warming, but we're looking at the global warming in the context of an icehouse Earth. The Earth's climate has alternated between ice house and greenhouse. The Earth is currently in one of his icehouse states, which is the climate's state characterized by permanent ice at both poles, it's called bipolar. Human evolution has taken place into this icehouse states with permanent ice at both poles. Civilization, we're going from the biological to the historical part. Civilizations arose during its most recent interglacial phase. However, icehouse states are much rarer than greenhouse states, only about 15 percent of the Earth history. Therefore we are today in a comparatively rare state and we don't have many examples of what happened in this rare state in the past. This makes it difficult to predict what effect warming would have in the future. If we look at the relatively recent past, temperature records that we observe show systematic fluctuations in greenhouse gases, which are in close sync with changes in the continental ice volume, the sea level, and the ocean temperature. Recent ice core records show that during the past 800,000 years, the Earth's atmospheric CO2 concentration was less than 300 parts per million. Currently, the CO2 concentration has grown from 280 parts per million in 1750 to 420 parts per million in 2019 and climbing. We have a lot of good data about the latest icehouse Earth. We understand greenhouse Earth much less well. However, what happened in the distant past for which we have less information may be of greater relevance to us, and why is that? Because the atmospheric CO2 contents have already surpassed by about 35 percent those that applied during the latest glacial-interglacial cycles. One more thing, large abrupt climate changes have repeatedly affected all of the Earth, sometimes reaching changes in temperature anomalies as large as 10 centigrades over 10, 20 years. The point is, catastrophic events such as meteorites, are not needed in order to trigger abrupt climate changes. Surprising new findings suggest that gradual changes can push the Earth beyond threshold, beyond which there is a runaway increase or decrease in temperature. The evidence that we have at the moment suggests that abrupt climate changes are possible and not very unlikely, I'm guarding my words here. It is not a central scenario, but it is not something that can re-write off is a really meditative event. Abrupt climate changes can make adaptation extremely difficult and they can disrupt the ecosystem severely. The interesting thing is that when we look at past cases of abrupt climate changes, we tend to find them associated with strong forcing, remember that forcing is a difference energy in, energy out of a climate system. Therefore, the fear is that greenhouse emissions, may increase the possibility of large, abrupt climate changes. To be totally clear, it is difficult enough to understand past climate, understanding abrupt climate changes is even more difficult, so everything should be taken with care and with a pinch of salt. However, the importance of abrupt climate changes is so big that we're going to look at them in greater detail.
