Hi. I'm Mary Sheehan, and we're going to talk about how the climate is changing. In order to talk about the health impacts of climate change and extreme weather on human populations, and how local decision-makers can be prepared in local communities be ready and resilient, we'll first start with some background on the science of climate change. So, I will go over some climate change concepts and definitions that we'll use throughout the course, and walk through some of the key findings of the most recent report of the Intergovernmental Panel on Climate Change or IPCC, the UN affiliated body tasked with monitoring our changing climate. First, we'll define some terms. So, what is the difference between weather and climate? A quote attributed to American author, Mark Twain, goes that, "Climate is what you expect and weather is what you get." But a more formal definition would be that, weather describes atmospheric conditions over a short period of hours, days or weeks, like the rainy day captured in the photo, whereas climate on the other hand, denotes trends usually over several decades. You can see evidence of climate trends in the map which shows changes in rainfall across the world over the last century, with the green to blue areas indicating increasing precipitation, and the orange to pink areas indicating declines in precipitation. When we speak of change in this longer-term concept of climate, it's helpful to have a definition. The IPCC defines climate change as a detectable, measurable change in the mean and/or variability of major climate properties, like temperature and precipitation. One that persists for an extended periods, so typically, decades or longer, and when that occurs due to either natural variability or as a result of human activity or both. That humans could have an impact on the climate is not a new idea. The concept of a greenhouse effect is more than a 150 years old. It was demonstrated in the 1850s when it was shown that water vapor in the atmosphere could absorb radiation. Later, the greenhouse effect was further elaborated when carbon dioxide emitted by coal burning was hypothesized to augmented and carbon dioxide to act as a greenhouse gas, warming the earth. In 1958, Keeling took this concept a step further and began measuring carbon dioxide at Mauna Loa in Hawaii. Here's the measurements, now known as the Keeling curve that you can see in the graph with the red curve, show that CO2 has steadily increased in the atmosphere since the industrial period. For comparison, the graph with the blue lines shows a reconstruction of carbon dioxide in the atmosphere over the course of geological time, starting 400,000 years ago. The CO2 in the atmosphere ranged from about 180 to about 280 parts per million. As that graph shows, at around the time of the Industrial Revolution, with more intensive burning of fossil fuels, the concentration of carbon dioxide in the atmosphere has substantially increased. As of 2017, the level was over 400 parts per million. This diagram shows how the carbon dioxide in the atmosphere acts, like the glass ceiling of a greenhouse trapping some of the incoming solar radiation which warms up the earth as heat. As more carbon dioxide is admitted, the greenhouse effect is enhanced and more heat is trapped further warming the earth, both the land and also oceans which absorb much of the heat. The difference between the incoming sunlight absorbed by earth and that radiated back out, is known as radiative forcing. But, carbon dioxide which comes from burning fossil fuels but also changes in forest and other vegetative cover, is not the only gas with potential to create a greenhouse effect. The greenhouse gases also include methane, for example from natural gas leaks and flares and livestock, as well as nitrous oxide from fertilizer, fluorinated gases from air conditioning and others. Whoever is shown in the chart, their potency as a greenhouse gas varies, with methane, nitrous oxides, and fluorinated gasses, all stronger heating potential than carbon dioxide. The graph shows the impact on radiative forcing also called warming influence, of the different greenhouse gases over time. Because it is so prevalent in the atmosphere, carbon dioxide which is shown in green, has the largest warming influence followed by methane in the orange red. Perhaps most importantly however, all of these gases are long lived in the atmosphere up to centuries or longer. Which means that the carbon dioxide and other gases emitted today, will likely still be in the atmosphere, and still be adding to the greenhouse effects and driving additional warming on earth for many generations to come. This underlines the idea that greenhouse gas-driven warming and the changes in the climate it creates, is a long-term challenge. A changing climate is not only a long-term challenge, is also a global one. Greenhouse gases are emitted by many countries around the world as shown in the pie chart. In particular, some of the largest enriches like the US and the EU countries, but also others such as China, Russia, and India. To create an international forum for cooperation on this global problem in 1992, the UN Framework Convention on Climate Change was set up, with the goal of minimizing net global warming. The specific target of two degrees Celsius of warming, and this is compared to the industrial period, were set in 2010 and in 2015, it was reinforced in the Paris Agreement. An agreement which also set the aspirational goal of endeavoring to do better than this by limiting warming to 1.5 degrees Celsius if possible. A few years earlier, the IPCC or the Intergovernmental Panel on Climate Change was established under the aegis of several UN organizations, with a goal to systematically review climate science, assess climate impacts and formulate responses. The IPCC is comprised of over 3,000 scientists from most world countries, and issues synthesis assessment reports periodically. In this course, we'll be relying on the findings of the fifth assessment report or what's called AR5. The most recent report which was published in 2013-2014. So, what does the IPCC's, AR5, report conclude? Well, first, the report concludes that compared with the pre-industrial era, warming of the earth's climate system is unequivocal. It further notes that, surface temperatures have increased around the globe about 0.8 degrees Celsius, which is about 1.4 degrees Fahrenheit. The surface layer of the ocean has warmed by about 0.4 degrees Celsius or 0.7 degrees Fahrenheit. The average sea level has increased by nearly 20 centimeters or about eight or nine inches. Many of these changes are unprecedented over millennia. The report concludes that, a human role is extremely likely. So, we'll look briefly at these points. In terms of heat, earth's recent warming can be seen in the graph, which shows the difference in global surface temperatures from a baseline average since the latter part of the century. Today's average temperatures are the warmest ever experienced by modern civilization. This warming is unevenly distributed over the earth and critically, CO2 emissions already emitted into the atmosphere, are sufficient for roughly a similar amount of additional warming. In other words, even if all carbon dioxide emissions stopped today, the earth would still continue to warm. This means that, not only is it urgent to take mitigation action, that is, reduced greenhouse gas emissions, but it will be important to learn to adapt to a warmer world. In terms of precipitation, because warmer air holds more water, the amount of rain and in particular, the intensity of precipitation has increased in many parts of the world over the last century. While it is difficult to attribute any particular storm to climate change, climate conditions make intense storms such as cyclones, more likely to occur and more likely to be extreme. For example, the 2017 Atlantic storm season, was among the most active with Hurricane Irma for example, the strongest ever recorded in the Atlantic. Because about 90 percent of the additional energy from the greenhouse effect is stored in the earth's oceans, these waters are becoming warmer and changing in chemical composition. With a warming ocean and a warmer earth surface, glaciers have also shrunk worldwide. These combined effects have led to sea level rise, which is similarly, unevenly felt across the globe. Finally, to help put into perspective the impact of human activity on global warming and a changing climate, the graph on the left shows in green some of the natural factors that have contributed to climate variability such as, solar activity and volcanic eruptions. The line in red, is the impact of anthropogenic or human driven greenhouse gas radiative forcing. When these factors are combined, the result looks like the graph on the right which indicates in green, the impact of natural factors only, and purple the impact of natural plus human factors. As you can see, the purple shading tracks very well with the actual observed temperature rise which is shown in the black line. So, in summary, climate change is a change in the mean and variability of climate parameters, whether driven by natural or human causes. Human activity since the industrial revolution has contributed to increased carbon dioxide and other gases in the atmosphere which have a greenhouse effect. Earth's surface temperature has increased and sea levels have risen, although these changes are unevenly distributed across the globe. Many of these climate changes are unprecedented over millennia. A human role in these changes is extremely likely. Finally, greenhouse gases are long lived in the atmosphere. Those emitted today will warm the earth for generations to come.
