First, let's define weather as opposed to climate. Weather is the state of the atmosphere at a particular time and location. So today, I'm in New Haven, Connecticut. The temperature right now is approximately 65 degrees Fahrenheit. It's not raining. The wind speed is light roughly five miles per hour and the relative humidity is roughly 50 percent. So that's the weather in New Haven, Connecticut today. On the other hand, climate is the long-term average weather for a particular location such as New Haven. Often, that average is over a 30-year period but that's not a hard-and-fast rule. In a nutshell, climate is what you expect, weather is what you actually get. So let's take a look at that a little further for New Haven. So this graph shows the average high and low temperatures in New Haven, Connecticut. Between 1980 and 2016 for each day of the year. So for example, on July 21st, we expect an average high of 82 degrees Fahrenheit. That's what the climate is. That's the average of those temperatures on July 21st, over that period of time from 1980 to 2016. However, we know that any particular July 21st, the high temperature is likely to not be exactly 82 degrees Fahrenheit. It may be much higher than that. It may be 95 degrees Fahrenheit or may be much lower, may be only 72 degrees Fahrenheit. So we know there's a lot of natural variability but on the other hand, there is a climate, there is an average weather. Let's also define climate change as any systematic change in the long-term statistics of climate elements sustained over several decades or longer. So the important point here is that climate change is a sustained change. We always see variability over a few year period that could be due to an El Niño event. It could be due to a major volcanic eruption. Could be due to minor changes in solar radiation. But then things go back to what the long-term average had been. When there's climate change, it's the long-term average that actually changes, not the change over just a few years. I also need to introduce the Intergovernmental Panel on Climate Change or IPCC. This is the authoritative international body that assesses the science related to climate change. The IPCC does not itself conduct research rather it does a comprehensive assessment of the published literature. This is done by literally hundreds of scientists. Every three or four or five years, they publish a major report. The fifth assessment report or AR5 was published in 2013-14 and each report consists of three subreports called Working Group Reports. Working group one is the physical science basis. Two, is the impacts adaptation and vulnerability and three, is mitigation of climate change. In addition to the assessment reports, the IPCC publishes what are called special reports and you've probably heard about the special report on global warming of 1.5 degrees centigrade that was published in the fall of 2018. This report compared the effects of an increase in global mean temperature of 1.5 degrees versus two degrees centigrade over pre-industrial levels. The Paris Agreement has set 1.5 degrees as an aspirational goal and two degrees as a firm goal. For each report, the IPCC produces a summary for policymakers or SPM and this Summary has actually gone over with a fine tooth comb by representatives of the world's governments. So they actually do line-by-line discussion every word in consultation with the authors and make edits. So in the end, we have a useful scientific summary. But it tends to be conserved for due to the need for consensus about the wording. So I think it's fair to say that if the IPCC SPM says that something is bad related to climate change or that something bad is going to happen that it will be at least that bad. So here's a conclusion of the latest IPCC report. Warming of the climate system is unequivocal and since the 1950s, many of the observed changes are unprecedented over decades to millennia. The atmosphere and oceans have warmed, the amounts of snow and ice have diminished, and sea level has risen. So let's first look at the warming of the atmosphere and ocean. Now we're back to Connecticut. This is Connecticut as a whole not just New Haven, Connecticut and here we're looking at the Connecticut average temperature for each year between 1895 and 2017 and the first thing that you might note is that there's a lot of variability from year to year. You actually know that just from living. However, a second thing to note if you look at the blue line is that there's a definite upward trend. So that back in 1895, the average temperature in Connecticut was less than 47 degrees Fahrenheit. Whereas in 2017, it was over 49 degrees Fahrenheit and approaching 50 degrees Fahrenheit. So there's been a definite trend of roughly 0.3 degrees Fahrenheit per decade of increasing temperature in Connecticut. Now I need to go on. I need to define temperature anomaly or departure. Those are the same thing and it's the difference between the temperature at a time and location of interests and a reference temperature. Usually the mean temperature over previous multi-decadal range at that same location. So this graph shows trends in annual mean global temperature now in degrees Fahrenheit and its trends and the anomaly from 1880-2016 using a 1961-1990 base period. So what that means is you take the average global temperature for 1961-1990 and use that as the base period for comparison with each year. So that is set 1961-1990 average is set at zero and we're looking at the departure from that average for each year between 1880 and 2016 and this was done for six different independent datasets and what's striking is how consistent the data are and you can see the increase in average mean global temperature over the years and the increase started to really take off around 1980 or so. This is looking at the world in a different way. Instead of averaging the global temperature as a whole, we're dividing the earth up into different grid cells and we're looking at the average, the change in temperature between 1901-1960 versus 1986-2015 in each of those grid cells and so it's also important to notice what the color-coding is. Where the darker red we see, the more the increase in average temperature between 1901 and 1960 versus 1986-2015. On the other hand, a decrease in temperature is represented by blue and obviously, you don't see much blue on this map. It's all pretty much all orange or red and the other thing to note is that the temperature increase is variable across the globe and that the biggest increase has occurred up in the Arctic regions. So now let's turn to the ocean temperature. So it turns out that much of the heat trapped by greenhouse gases, we'll talk about greenhouse gases a little later, heats the oceans because they have an enormous heat capacity and this actually represents a massive accumulation of energy in the oceans and that's very important because that energy ends up fueling tropical cyclones and hurricanes and we'll also talk more about that later. So let's focus on this slide. Let's first of all focus on the top graph and this shows annual global mean upper ocean heat content between 1950 and the near present and a couple of things to note here. First, is why upper ocean? So it turns out that because the ocean is so vast and so deep, that we see upper ocean heating occur, and this is intuitive, expect this to happen, first and then the heat slowly diffuses down and then you start seeing heating of the mid-depth and deep ocean. So we see that this isn't looking at ocean temperature, it's looking at heat content, but we see that there's been since around 1980 or so, there's been a fairly rapid and substantial increase in ocean heat content. Then the lower graph shows the mid-depth and deep ocean heat content and we see that those have also been increasing albeit at a slower rate and also starting a bit later in calendar time because it took some time for the heat as I mentioned to diffuse down to those depths of the ocean.
