So first let's define albedo, which is also reflectivity. Often Alpha is the symbol that's used for that. Albedo is the fraction of solar radiation reflected and not absorbed by an object. So a black surface is a perfect absorber and has a Alpha or reflectivity of zero and a white surface is a perfect reflector and it has a Alpha of one. To give this some contexts, the mean Alpha of earth is 0.3. The Alpha of the open ocean which is dark is low, it's 0.06. Alpha of bare ice is much higher, it's 0.5. Alpha of ice with snow which is very white is 0.9. So it's very high. So snow then is almost a perfect reflector and reflects most of the solar radiation that hits it as opposed to it being absorbed. Now, let's define a radiative or climate forcing which we'll call an RF. An RF is a change in the balance between incoming solar radiation and outgoing infrared radiation as a result of a change imposed on the planet. A positive RF warms the climate and negative RF cools the climate. Once an RF is imposed, it takes time before the climate adjusts back to equilibrium by warming or cooling, depending on the type of RF. So what types of RF could produce climate change? They could be natural forcings. So there could be natural long-term changes in solar emission, slow changes in the Earth's orbit could cause climate change, volcanic eruptions if they're sustained over a number of years could cause climate change, and there could be permanent changes and natural cycles such as El Nino La Nina. As we've seen already, there also anthropogenic forcings and those are actually happening, the fossil fuel burning, the land use changes, and the animal agriculture are the three major ones. Now when a forcing an RF is imposed on the planet, we don't see an instantaneous change in the climate. It takes time for the climate to reach a new equilibrium temperature and this phenomenon is called thermal inertia. Most of the thermal inertia comes is due to the oceans. As we've already talked about, the oceans have enormous heat capacity and it takes a really long time, if it's a positive RF for example, to warm the oceans or if it's a negative RF to cool the oceans back to equilibrium. This could take up to thousands of years. So it turns out that even if we stopped emitting greenhouse gases today, we're already committed to about another half a degree centigrade of further warming due to the greenhouse gases that have already been emitted. So another conclusion of the fifth assessment report of the IPCC, it is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century. The evidence for that we've actually gone through a lot of the evidence, but there is a three-fold evidence. First, there's a strong theoretical rationale based on the physics of the greenhouse effect and we've talked about that. Second, there's a strong correlation between atmospheric carbon dioxide and temperature throughout Earth's history. So paleoclimatologists have figured that out. Finally, climate models must include anthropogenic changes in greenhouse gases and aerosols to accurately reflect the actual observed changes. So let's briefly go through the second and third of these pieces of evidence. So first, this is from paleoclimatologists who have generated this. So we're looking here at the last half million years or so and blue represents temperature and the red represents atmospheric carbon dioxide and you could see how closely they've tracked over the last half million years, very close correlation. That doesn't prove causality. Correlation doesn't prove causality, but there are other reasons to believe that we don't have time to go into that this does represent causality. So on this slide, let's focus on these two graphs and I'll walk you through what we're seeing here. So we're looking at time trends from 1860 to the near present and we're looking at temperature anomaly, this is global temperature comparator in 1951 to 2010 baseline. So that's what zero, the average temperature from 1951 to 2010 would be set to zero for the axis. The black is the actual observed global mean temperatures for each year. In this top graph, the climate models are looking at just the natural forcings, they're not including the anthropogenic or human forcings. So there are two sets of climate models, the CMIP3 and CMIP5. You can see that if you only consider the natural forcing, the models don't do a good job at predicting what's actually happening with the global mean temperature. On the other hand, if we now put the human forcings into the models, we see that they do a very good job of tracking the actual observations. So this is a very strong piece of evidence in favor of the human anthropogenic forcings being responsible for the climate change that we have observed. So in summary here, this is looking at radiative forcing from 1750 to 2011. Human-caused is virtually all of it, solar a little bit, volcanic actually a slight negative forcing. How much warming has there been? According to the IPCC 2018 special report, human activities are estimated to have caused approximately one degrees centigrade of global warming above pre-industrial levels with a likely range of 0.8 degrees centigrade to 1.2 degrees centigrade. Okay. So now let's turn briefly to the concept of climate feedback defined as an interaction in which a perturbation in one climate quantity causes a change in a second which ultimately either amplifies that would be a positive feedback or attenuates that would be a negative feedback the initial effects. These feedbacks are important because the positive feedbacks may actually contribute more to warming than the greenhouse effect due to greenhouse gas emissions alone. So it's possible that we have these amplifying effects that are actually more important than the direct greenhouse gas effects. There are a lot of possible feedbacks, I'm just going to give examples of three of them that we think are actually happening. So the first is the sea ice albedo feedback. So here we know that temperatures are rising, especially in the Arctic. That's causing sea ice to melt we know that, and because sea ice is melting, the surface albedo decreases as the sea ice is very white, it has a high albedo. On the other hand, the open ocean is much darker and has a low albedo. So the ocean absorbs more heat, causes the temperatures to rise more and we have this vicious cycle. This slide shows sea ice and dark ocean water. You could see the stark difference in albedo between the two. The second feedback I wanted to mention is the water vapor feedback. So here we have our increasing CO2 causing global warming, that increases evaporation, so we have increased water vapor in the atmosphere. Remember that water vapor is in itself a greenhouse gas, so therefore having more water vapor in the atmosphere increases temperatures, more global warming, we have another vicious cycle. So here it's not anthropogenic activity that's directly putting more water vapor into the atmosphere, this is a feedback that we're putting more CO2 and other greenhouse gases into the atmosphere that are causing the warming that then causes more H2O to be in the atmosphere, which causes more warming. So then the third and last feedback I wanted to mention is the permafrost thaw carbon feedback. So again, this is important especially in the Arctic which is warming the fastest, we have rising northern temperatures which causes increase permafrost thaw. I should mention permafrost is frozen ground that contains dead organic plant matter that does not normally decay when it's frozen. But now we have increased permafrost thaw, increased microbial primary production meaning that now that the permafrost is not frozen, microbes could start working and it will decay this organic matter and then both CO2 and methane, the two greenhouse gases are products of metabolism of these microbes. So that increases greenhouse gas release, causes more warming and we have another vicious cycle.
