Hello, and welcome back. I'm Kevin Anchukaitis. I'm an Associate Professor in the School of Geography and Development and in the Laboratory of Tree-Ring Research here at the University of Arizona. In this section, we're going to be talking about global warming, change in Earth's temperature due to human activities, and the role that CO2 plays in that warming both the observed warming, past variability and then what we expect for the future. So, all the energy in the air system, the energy that drives storms, that moves the winds, that moves ocean currents, that even powers the biosphere, that's responsible for the activities of the hydrosphere, the atmosphere, and yes, even the living things around us, comes from the sun. All that energy enters in through our atmosphere, which is mostly transparent to the sun's energy. That energy passes through that atmosphere, arrives at the surface of the Earth and is absorbed by the surface of the earth. The earth heats up and then it tries to return that energy to space. This time though, the atmosphere standing in the way. That atmosphere now, absorbs that energy as the earth tries to get rid of it and sends some of that energy back down to the surface of the planet. So, everything radiates some energy, you are radiating energy right now. I'm radiating energy right now. My morning cup of coffee was radiating energy. Most of the things in the earth system though, are radiating in a part of the electromagnetic spectrum that we can actually see, in the infrared or the thermal infrared. But with a special camera like this, you can see my morning cup of coffee actually, sending out all that energy. So, the earth's surface is doing the same thing. It's liberating all this energy trying to send it back into space. But standing in the way are a number of what we call, greenhouse gases. And one of the most important of these is carbon dioxide. So, we have a carbon attached to oxygen and this molecule is very important in the earth's atmosphere. And the reason is that the structure of carbon dioxide is such, that it can intercept and absorb and then re-radiate that infrared energy that the earth's surface is trying to get rid of. And so, just like say, a piano string that vibrates at a certain frequency, CO2 vibrates at the frequency of the earth's of infrared energy. So, it absorbs that energy and sends it back to add even more energy to the surface. And so, you get some energy directly from the sun and then that additional energy from the greenhouse atmosphere and this is what we call, the greenhouse effect. The fact that the atmosphere, our atmosphere is sending energy back down to warm the surface even more. And so, the surface warms up more, it tries to radiate more energy back into space and this continues until the amount of energy coming into the system is balanced by the amount of energy going out, which gives us our surface temperature. Now, this is a natural effect. Without the greenhouse effect, we would have a cold icy lifeless planet. And so, the natural greenhouse effect is what gives us our current state of the climate. Now, normally the amount of carbon in the atmosphere is balanced by all these processes on the land, and in the atmosphere, and in the ocean. So, there's a certain amount of carbon in the atmosphere, the land biota, the plants absorb that. Of course, as they decay, they send some of that carbon back out, much of that carbon enters the ocean but also diffuses back out. And so, there is within the air system, a natural cycle that moves carbon around in this system from the atmosphere to the biosphere to the ocean and back again. Now, what's changed in the last few hundred years, is this part of the cycle, human emissions. What we've done is we've gone and found the source of very old carbon, oil, coal, natural gas. We've dug it up to drive our modern civilization. And in the process, we have added a new flux, a new source of carbon to the atmosphere that wasn't there several hundred years ago. Every year we put about 35 gigatons of carbon dioxide into the atmosphere. About half of that stays there. The land takes up some, some of this goes back into the ocean. About half of all the carbon dioxide we put into the atmosphere, stays there increasing year by year, the amount of carbon dioxide in the atmosphere and therefore, the strength of the greenhouse effect. What we find ourselves with today, is an intensified greenhouse effect where the quantities of CO2 but also other gases like methane have increased. This traps more of that earth energy in your system and as a consequence the land and the ocean both heat up, much of that energy goes into the ocean. Some of the energy is used to melt ice and that energy raises the temperature of the land and the atmosphere. And this is our intensified greenhouse gas or our intensified greenhouse effect. Less energy escapes from the earth and so, the surface heats up. So, I said that we've changed the amount of CO2 in the atmosphere, by digging up fossil fuels, burning them and emitting that CO2 to the atmosphere but just how much have we changed it? Much of what we know about how we've changed the atmosphere as greenhouse gas composition is thanks to, Charles David Keeling who in the 1950's developed a very precise way to measure the amount of CO2 in the atmosphere. He went as far as he could away from sources of industrial CO2, so he could measure the free atmosphere and found his way to the volcanoes of Hawaii. And here, he started measuring atmospheric CO2 at Mauna Loa Observatory. And he observed two things within just the first few months and years of observations. One was that there was a regular annual cycle of CO2 in the atmosphere and we now know that this is because as the earth greens up in spring, the Northern hemisphere greens up in spring and summer, that the biosphere draws down CO2 to put on new leaves and new growth. And then as the biosphere goes into Northern hemisphere winter and goes into dormancy and leaves fall off the trees that CO2 re-enters the atmosphere. And so the Northern hemisphere breathes in and out, with regular consistency and that's the red cycle you see there. But in addition to this regular cycle, is a trend an upward trend. And this trend has continued from the moment that Keeling started measuring until today and it continues and will continue for the foreseeable future. And this is all due to the emissions of greenhouse gases by humans, Anthropogenic greenhouse gases. You can see that we've reached now, over 400 parts per million of the atmosphere is composed of CO2. When Charles Keeling started measuring it, it was less than 320. Now, that only takes us back to the 1950's, but we can use ice cores to find out what CO2 was like, before Keeling started measuring atmospheric CO2. Drilling in to these ice cores and extracting the old atmosphere from the bubbles, those trapped in that ice, allows us to measure ancient atmospheric composition just to know how much CO2 was in the air at that time. And if we do this, we can go back into the 1700s and see what CO2 was like, then. And what we see is that before the industrial revolution of the late 1700s and the early 1800s, the carbon dioxide was only about 200 and 70 or 280 parts per million of the atmosphere. That as the industrial revolution began, and as there were increasing amounts of industry transportation and the burning of fossil fuel. The fossil fuels have risen steadily to their current level above 400 parts per million. And all of this rise, this rise from 275 in the 1700s to over 400 today, is due to the human emission of greenhouse gases. We can go back even further, if we use the shells of these marine foraminifera, and we measure the chemistry of those shells. And what those shells reveal to us is that, if you go back two million years ago, there is now more CO2 in the atmosphere over 400 parts per million than there was over the last two million years. And all of those changes have been incredibly rapid over just a few hundred years and all of it is due to human activity. What's the consequence of increasing the amount of CO2 increasing greenhouse gases in the atmosphere and warming up the surfaces that we observe more and more temperature records and a steady trend upwards in global mean temperature. This is a plot of measured temperatures or observed temperatures over the last century or more since 1880. And what you see is year to year variability and even variability from decades to decades, but an upward trend that becomes particularly obvious in the 1970's with a consequence that we've warmed more than 1 degrees Celsius over the last century or so. So much so that the year 2016 was the warmest year on record, beating out the record for 2015 which was the previous warmest year on record. Now, when we say warmest year on record, we're obviously limited to the period of which we've been measuring global temperatures, When we had weather stations. To put this into a longer term context though, we can use tree rings as we discussed in the previous section to actually reconstruct temperature back in the past. And so to do this, we can go to the very northern tree line, go up to Alaska or the northern part of Russia, find trees whose growth is limited by the temperatures of the summer. We can take the course from those trees, measure their rings. And a wider ring means it was warmer, and a narrow ring in these environments it means it was colder. We can collect trees like this from all around the northern tree line. We can collect trees like this from very high mountains, measure their rings and develop a long term picture of the growth of these temperature limited trees. We can then use statistical models to estimate the temperature back through time, converting those measurements of ring wits into actual estimates of past temperature. Now when we do this, what we see is that we can go back much further than the instrumental record which is what's shown here in red. In fact, we can go back to the year 750 AD or perhaps even earlier. And when we do that what we see is that while there have been swings in temperature, year to year even decade to decade, perhaps a slightly warmer period back here in the medieval era and a colder period here over a few hundred years of the little ice age. What we see is that our current temperatures appear in this right hand corner, are now warmer than any year in the northern hemisphere in over 1,000 years. And this rise here, this very steep rise is due to human emissions of greenhouse gases. How are we sure it's us though? How do we know this isn't just some natural variability some natural cycle? Well, we can't really run multiple experiments on the earth. We only have one earth, things only happen one time. But what we can do is we can set up experiments using computer models of the earth's climate system. And because we can simulate the physics and the chemistry and the biology of the earth system within these computer models, these very sophisticated computer models, we can actually run experiments to try and see what would have happened if we hadn't emitted all that CO2. What would the temperature history of the globe been if we hadn't had that increase in CO2? So we can use these computer models, these climate models to run the kind of experiments we can't really run on the earth system. Now we can tell the earth system models, the computer models, all about things like the amount of solar insulation, radiation coming into the earth system. We can tell it about when large volcanoes went off and helped cool briefly the planet. And of course we can tell it how much CO2 has been emitted into the atmosphere. So if you run this model with just the natural forcings, just the solar variability, just the volcanoes, and you compare it to what actually happened, this is what you see. Maybe there's some agreement early in the 1900s. But by the time you reach the middle of last century, what actually happened and what the computer simulate start to diverge. Using only natural forcings, using only the sun, only volcanic eruptions, you can explain this rise in temperature. We would have expected temperatures to remain mostly flat. If you add CO2 into your model though and actually tell the computer exactly how much CO2 was added each year to the atmosphere, then you can see in this red line that the computer can reproduce that long term trend. Now what is this telling us? This is telling us that only models that include human emissions of CO2, only those models that include the human impacts on the earth system, only those models can reproduce what actually happened. And this gives us considerable confidence to say that these trends particularly since the 1960s and 70s are due to the emissions of CO2 and the increase in the greenhouse gases in the atmosphere. So if it's only those models that include CO2 that can reproduce what actually happened, we have increased confidence that it really is us. It really is our activities, our emissions of those greenhouse gases that are causing global warming. So what does the future hold then if this is the past and the present? What might we expect for the future? Well, we can use those same models to make predictions for how things will change in the future. Now one thing we don't know is exactly how much CO2 we'll emit going forward. Right now we emit almost 40 gigatons of CO2 per year. We don't necessarily know if we'll continue to do that but if we do if we continue along our business as usual path, if we continue to dig out more and more fossil fuels and burn them and don't convert to more efficient forms of energy, then we might expect a trajectory of emissions that warms the planet by as much or more than 4 degrees Celsius. That's what you see in the red line. But maybe we choose a different path, maybe we choose a path of greater energy efficiency of actually trying to capture CO2 from the atmosphere and sequester it. Under that scenario where emissions don't rise and maybe we even find ways to capture CO2 from the atmosphere, the blue line shows what we might expect a leveling off of this temperature rise and maybe even a stabilization of temperatures around where we are now. So the big uncertainties for the future are those that are associated with exactly how much CO2 we'll emit, exactly how much of the greenhouse effect will be intensified by our activities. Will we choose a path of high intensity emissions or will we find a path that reduces those emissions and stabilizes temperatures. Temperature rise, global warming, increase in energy and the system affects nearly all human and natural systems, and we can already start to see the signs of this on the landscape. We already observe that the air temperature has risen as I showed you. Temperatures over land as well as temperatures over the ocean. The temperature of the ocean itself has risen. The amount of heat stored in the ocean water has gone up. We've seen reductions in glacier volume, reductions in sea ice, reductions in annual snow cover. At the same time we've seen an increase in water vapor as the atmosphere warms up and can hold more water. All these things are signs of a climate that is warming right now, and these are some of the impacts that will continue to affect both human systems, our cities and our communities and our infrastructure as well as natural systems, forests and the landscapes here and desert southwest as well. So what do we know? We know that the earth is warming up. We actually know that not only has it warmed degrees C over the last century or so, but we're now warmer than we've been in over a millennium. We know that this warming is caused by human emissions of greenhouse gases particularly CO2. We also know that CO2 and warming have already affected the earth's system, multiple parts of the earth system, and will continue to have an impact on all human and natural systems. But one thing to take away from this also is that the amount of warming we'll see in the future is one of the biggest uncertainties and that's really up to us. How much more we will emit? How many more gigatons of CO2 will we put into the atmosphere. That's one of the big uncertainties of the future, and that's up to us.
