Global Change and Greenhouse Phenomenon

Global Change Basics:

I am going to discuss global change, global climate change, and the mechanics of global greenhouse gases and warming. First of all, what is global change? Global change is change that occurs in the chemical, biological, and physical properties of the planet. Global change is natural, and has been occurring for hundreds of millions of years. Planetary conditions such as temperature and carbon dioxide concentrations are not static. They definitely change without the influence of human beings. For example, in the recent past-- that is, geologically speaking-- roughly 30% of the Earth's surface was covered by ice. This most recent glacial period ended roughly 12,000 years ago.

However, in the past century, there have been global changes of all sorts. One prominent example is the increase in atmospheric carbon dioxide. It is widely agreed-upon that human beings have significantly impacted the atmospheric concentration of greenhouse gases such as carbon dioxide. What is the effect of the increase in carbon dioxide and other greenhouse gases? The Intergovernmental Panel on Climate Change, IPCC, an international group of scientists under the auspices of the United Nations, state that the temperature on Earth has been warmer in recent decades, and that humans are almost certainly the primary cause. Here is exactly what they said in September 2013 in the working group One Summary for policymakers. Warming of the climate system is unequivocal, and since the 1950s, many of the observed changes are unprecedented over decades to millennia. 

Weather is a short-term description of temperature, winds and rainfall. Climate is the average weather that occurs in a given region over a longer time period. Global climate change is a type of global change that refers to changes in the climate of Earth. Changes in climate can be categorized as natural or anthropogenic, meaning caused by humans.

Global warming refers specifically to one aspect of climate change- the warming of oceans, land masses and the atmosphere of Earth that takes place over longer time scales than the annual seasonal cycle of being warmer and cooler or being dryer and wetter. What is the source of this warming? The ultimate source of almost all energy on Earth is the sun. Incoming solar radiation consists primarily of ultraviolet and visible light. About 1/3 of this incoming solar radiation is reflected from the atmosphere, from clouds and from the surface of the planet back into space. The remaining solar radiation is absorbed by clouds, and by things on the surface of the planet. Thus, the clouds and planet's surface are warmed in this process.

The Earth's surface emits the absorbed solar energy as infrared radiation. Greenhouse gases surrounding the Earth absorb much of this radiation, emitting the infrared radiation back to the Earth. This is what we call the greenhouse effect. Quantities and processes that change the absorption and radiation of energy are drivers of climate change. Radiative forcing is a term that quantifies the energy flux to and from the Earth. Positive radiative forcing means the Earth will get warmer. Negative radiative forcing means the Earth will become cooler. There's a known quantity of energy from the sun that strikes the Earth each year.

The IPCC reports that radiative forcing is positive, meaning that there has been a positive flux of energy to Earth, and the Earth has been getting warmer. The largest contributor to the positive radiative forcing is the increase in atmospheric carbon dioxide since 1750. Earth now absorbs about two watts per meter squared per year more than it did in 1750. This means the greenhouse blanket of heat-trapping gases is thicker than it used to be, hence the warming. This may seem like a small net gain in energy, but over the global earth system, over time, this can add up to significant changes in temperature, resulting in a warmer Earth.

Global Change Effects:

Feedback loops are changes in one part of the system that influence another part of the system. Negative feedback loops return a system to its original state, while positive feedback loops move the system away from equilibrium. Clouds could play a role in both positive and negative feedback loops. For whatever reason, let's say there's been a slight increase in global temperature, the increased global temperature leads to increased evaporation of water. More water in the atmosphere may lead to formation of a particular high altitude cloud with low-albedo- that is low reflectivity. For a given amount of sunlight striking the Earth's atmosphere, more energy reaches the surface of the Earth. It doesn't reflect back into space. And the clouds also act as a blanket and absorb more outgoing infrared radiation that has reached the Earth, because the clouds contain water, and water is a weakly potent greenhouse gas. That leads to the Earth absorbing more infrared radiation and warm further, which leads to more evaporation of water and the formation of more clouds, and the positive feedback cycle continues, leading to warming of the earth even more.

But cloud formation can also contribute to a negative feedback system. So again, for whatever reason, let's say there has been a slight increase in global temperature. Increased global temperature evaporates more water. More water in the atmosphere may lead to the formation of low-lying, high-albedo clouds, high reflectivity. These clouds would reflect more incoming solar radiation that could lead to less warming of the Earth in the atmosphere by the Sun, which would lead to cooling. This is an example of a negative feedback system.

Let's consider a polar feedback loop. Warmer global temperatures melt polar sea ice. This melted ice results in an increase in the area of sea that is liquid, which is darker and has a lower albedo then sea ice. Remember, lower albedo means lower reflectivity, meaning it absorbs more energy from the Sun, thus this change results in warming the ocean and the ocean atmosphere. And that cycle continues.

 

Here's another polar feedback cycle. For whatever reason, there are warmer global temperatures. Warmer temperatures melt the permafrost Arctic tundra soils and deepen the active thawed layer in the Arctic. The fact that the soil is thawed for more months of the year, means that dead organic matter is available for decomposition for more of the year. Because the thawed permafrost is wet and poorly drained, most of the decomposition that takes place is going to be anaerobic, meaning that methane will be produced rather than carbon dioxide. As we said, methane is 25 times more potent as a greenhouse gas than carbon dioxide, so it will lead to more warming, which will lead to more melting of permafrost, which will lead to more anaerobic decomposition, and so on. You can see we would have a strong positive feedback system.

There are some negative feedback systems that could become quite prominent. So for example, increased atmospheric carbon dioxide concentrations will increase plant growth. Increased plant growth means that plants will take up more CO2 from the atmosphere, thereby decreasing the amount of carbon dioxide in the atmosphere, which will lead to global cooling. So that is potentially a prominent negative feedback cycle related to plant growth and carbon dioxide.

All of these cycles and many others are currently in play somewhere in the world or many places in the world. And some have received a great deal of attention and some are still maybe being understood and explored. All of these things work together. It's unclear which feedback cycles will become more dominant in a future scenario of higher CO2 concentrations and warmer temperatures. So it's really quite a challenge, and it's difficult to say which cycles will become the dominant cycles. Will we see more and more positive feedback cycles and runaway increases in temperature? Or will one of these or one of the negative feedback cycles that we haven't explored take a more important role in the future and bring things back? That's why there's so much uncertainty about what's going to happen in the future. And that's one of the reasons why this is such an exciting and challenging field to understand and to study. 

Greenhouse Gases:

Greenhouse gases are gases in the atmosphere that can absorb infrared radiation, the energy that radiates from the Earth and from the lowest level of the atmosphere, the troposphere. Let's review the major anthropogenic greenhouse gases. Global warming potential refers to the amount of infrared radiation that a given quantity of each gas will absorb or retain. It's a measure of how potent a greenhouse gas is. Rather than give you the actual value of absorption, we compare each greenhouse gas to carbon dioxide. This means carbon dioxide is given a global warming potential of one. Other gases will have a global warming potential for more or less than one. 

Carbon dioxide - CO2: The two major anthropogenic sources are the combustion of fossil fuels and the conversion of land resulting in the net loss of vegetation. And this includes deforestation. The global warming potential is one. 

Methane - CH4: The sources are anaerobic decomposition-- that is, decomposition without oxygen-- which occurs in waterlogged irrigated fields, such as rice paddies; the digestive tract of ruminant animals; and created wetlands; and landfills. The global warming potential of methane is 25. That is, it's 25 times more potent than carbon dioxide. 

Nitrous oxide - N2O: Sources are wet nitrate-rich soils, such as agricultural fields, and other flooded areas of land. The global warming potential for nitrous oxide is 125 to 300 times that of CO2. 

Chlorofluorocarbons - CFCs: These are made by humans for refrigerators and air conditioners. The global warming potential varies widely, depending on the particular CFC, from roughly 1,500 to over 10,000 times the global warming potential of carbon dioxide. 

Water - water vapor: It's a little strange to talk about water vapor as a greenhouse gas. It occurs naturally in evapo-transpiration, of course. But if humans are responsible for increasing the amount of evaporation, we can say then that humans are responsible for putting this greenhouse gas into the atmosphere in greater quantities. The global warming potential is less than one, and it varies. 

Current atmospheric carbon dioxide concentration is about 400 PPM. CO2 measurements from ice cores show that for the past 400,000 years, CO2 concentrations had not been above 300 PPM.

A sharp increase in global carbon dioxide concentrations occurred after 1950. This increase in carbon dioxide concentration coincides with increased burning of fossil fuels and net changes in land. Production of carbon dioxide has been greatest in the developed world. The 20% of the global population living in the developed world has produced roughly 3/4 of the additional carbon dioxide released to the atmosphere. Global temperatures have increased by 0.85 degrees C, roughly 1.5 degrees Fahrenheit from 1880 through 2012. There is a close correspondence between historic temperatures and CO2 concentrations. However, it's not clear if temperature is driving higher CO2 or if higher CO2 concentrations are driving temperature. This increase may seem insignificant - 1.5 degrees Fahrenheit-- but the warming is not evenly distributed around the globe. Northern regions of the globe, particularly the Arctic, have experienced more substantial increases in temperature since 1880. We also need to remember that even small amounts of warming could lead to feedback loops that would then hasten the rate of global warming.