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Part 2

Climate Change

Let’s find out what the models say. If nothing is done to curb emissions (under scenario Representative Concentration Pathways (RCPs) are used in climate modeling and research to provide projections of how greenhouse gas concentrations in the atmosphere will change between now and 2100. RCP8.5 is the most extreme scenario, assuming that emissions continue to rise throughout the 21st century.RCP 8.5), how much warmer could our planet be in about 300 years?

Between 7ºF and warmer.

+0ºF +0.0ºC
+10ºF +5.5ºC
+20ºF +11.1ºC
+30ºF +16.6ºC
It’s at least this much 7ºF

Chapter 04

The climate is always changing

Figure 8 Earth’s orbital eccentricity, axial tilt, and axial precession change in cycles. These cycles work together to trigger ice ages.

Earth’s orientation in space isn’t constant: our planet’s tilt, wobble, and the shape of its orbit around the sun cycles over the course of tens of thousands of years. These cycles are what cause the natural fluctuations between global ice ages and warmer periods. Our planet’s climate is also affected by volcanic activity and changes in how much energy from the sun hits the earth. One way that we know that the current warming is caused by human activity is because we are currently in a cycle that should be cooling the planet.

Video 2 The History of Climate Cycles (and the Woolly Rhino) Explained PBS Eons Video description: Throughout the Pleistocene Epoch, the range of the woolly rhino grew and shrank in sync with global climate. So what caused the climate -- and the range of the woolly rhino -- to cycle back and forth between such extremes?

Ice ages are controlled by how much sunlight is received by the Arctic region during summer.

In the early twentieth century, mathematician Milutin Milanković calculated (by hand) how the distribution of sunlight varies and speculated that variations in arctic sunlight caused the ice ages.

Video 3 A History of Earth’s Climate SciShow Video description: Earth had a climate long before we showed up and started noticing it and it's influenced by a whole series of cycles that have been churning along for hundreds of millions of years. In most cases those cycles will continue long after we're gone. A look at the history of climate change on Earth can give us some much needed perspective on our current climate dilemma because the surprising truth is, what we're experiencing now is different than anything this planet has encountered before. So, let's take a stroll down Climate History Lane and see if we can find some answers to a question that's been bugging Hank a lot lately - just how much hot water are we in?

The cause of these cyclic swings in temperature and the associated growth and retreat of great continental ice sheets was proposed by several scientists, notably by the Serbian mathematician Milutin Milanković in 1912. He recognized that the shape of Earth’s orbit around the sun varies cyclically over time—back and forth from more circular to more oval—with a period of about 100,000 years. Milanković also knew that Earth’s tilt with respect to the plane in which it orbits the sun wobbles over a cycle of 41,000 years, and that Earth’s rotation axis Precession is a change in the orientation of the rotational axis of a rotating body. Imagine a wobbling, spinning top.precesses like a top with periods of 19,000 and 23,000 years. These three factors—our planet’s tilt, wobble, and orbit—affect the way sunlight is distributed around the world, even though they hardly affect the total amount of sunlight received by the planet as a whole.

He speculated—correctly, it turns out—that ice ages are controlled by how much sunlight is received by the Arctic region during summer, and set about calculating this value from the basic laws of physics that control the earth’s orbit and rotation. After years of hand calculation, Milanković produced a curve showing how ice ages should behave. At that time, data such as those used to produce Figure 7 did not exist, and so there was only rough agreement with what little information there was. But today we know that the great ice ages were caused by the cycles computed by Milanković, though there are gaps in our understanding of the details of how Earth’s climate responded to these.

Figure 9 Until about 1900, the summer temperature in the Arctic was going down.

Until the industrial revolution, the Arctic had been cooling for many thousands of years.

Unimpeded, this mechanism would lead the earth toward another ice age.

This expected cooling is illustrated in Figure 9, which zooms in on the last 2,000 years of temperatures in the Arctic. The slow, steady cooling trend from the beginning of the record to around 1700–1800 CE probably reflects the slow decline in sunlight reaching the Arctic due to the Milanković orbital cycles. Unimpeded, this mechanism would lead the earth toward another ice age, with continental ice sheets beginning to grow thousands of years from now. But note the strong uptick in temperature toward the end of the record, particularly after about 1900. This is quite unusual by the standards of the last few thousand years and reflects the increase in carbon dioxide and other greenhouse gases brought about by humanity’s rapid consumption of fossil fuels. We are certain that this increase in CO2 concentrations was caused by human activities because the isotopes of carbon in ice show that it comes from fossil fuel burning and the clearing of forests. Over the course of a few hundred years, humans rapidly burned fossil fuels that nature created over tens of millions of years.

Chapter 05

How much of the CO2 increase is natural?

Figure 10 Current CO2 levels are well beyond anything our planet has seen for the past 800,000 years.

So, the evidence suggests that the warming of the last one hundred years is unusual compared to the last few thousand years and is almost certainly caused by higher concentrations of CO2. But could the increased CO2 concentrations themselves be natural?

Video 4 Modeling the Future of the Greenland Ice Sheet NASA Climate Change Video description: Scientists at the University of Alaska Fairbanks’ Geophysical Institute used data from NASA’s Operation IceBridge to develop a more accurate model of how the Greenland Ice Sheet might respond to climate change in the future, finding that it could generate more sea level rise than previously thought. In the next 50 years, the model shows that melting at the present rate could contribute one to four inches to global sea level rise. This number jumps to five to13 inches by 2100 and 19 to 63 inches by 2200. These numbers are considerably higher than previous estimates, which forecasted up to 35 inches of sea level rise by 2200.

06a What are climate models?

The real issue, of course, is what will happen in the future. Although ultimately we want to know what the human and monetary risks are, we need to start with something simpler: how global temperature will evolve going forward. But the Earth’s climate system is immensely, almost overwhelmingly complex. To deal with this complexity, scientists have created computer models to map these interactions and simulate how the Earth might respond under different scenarios and assumptions—and we’re getting an increasingly clear picture of what kind of temperature changes we can expect in the coming decades.

Our climate system is incredibly complex. Features like clouds, ice, and vegetation must be handled approximately and that, among other things, introduces uncertainty in our models.

06b Uncertainty

Scientists face tremendous challenges when attempting to model the Earth’s climate system. While there is a solid understanding of how many parts of the system work, our incomplete understanding of several aspects of the highly complex climate system introduces uncertainty into our attempts to forecast how climate will change.

Figure 11 It takes time for the temperature of a system to increase after an initial burst of warmth.

Another source of uncertainty is the response of the deep ocean to climate change. The oceans act as a buffer to temperature change and delay the response of global temperature to increasing greenhouse gases. Here is a good way to think about the effect of the oceans. Suppose we have a sealed glass cylinder containing equal volumes of air and water. If it is just sitting at rest with no energy going in or out through the walls of the container, the air and water will settle down to the same temperature. Add enough black dye to the water to make it opaque and shine a powerful flashlight down through the glass top of the cylinder. The light passes through the air but is absorbed at the very top of the water, heating it. So the top of the water warms up, and since that is the part that is in contact with the air, the air warms up too. But the water below the surface is not heated by the light, which never makes it down below the surface, so it remains at the temperature it had before. But slowly—very slowly—the warmth of the surface water is diffused down into the deep water and this both warms the deep water and cools the surface water and with it, the air.

Thus after we turn on the flashlight there will be an initial fast warming of the air and surface water, followed by a very slow increase in the temperature of the whole system. Eventually, the water and air will reach a new, warmer temperature. How long it takes to do so will depend on how rapidly heat diffuses downward into the deep water.

Video 5 The Last Time the Globe Warmed PBS Eons Video description: Imagine an enormous, lush rainforest teeming with the Arctic. Well there was a time -- and not too long ago -- when the world warmed more than any human has ever seen. (So far)

Our models could account for the lag between heat input and temperature change in the real world if we had a simple theory for how heat penetrates the ocean depths. We know that heat is mixed rapidly downward to a depth of between 20 and 150 meters (60 and 500 feet), depending on location and time of year. If heat did not penetrate deeper, then the 20–150 meter penetration would give a lag of around two years. But we know from measurements that heat manages to circulate much deeper in the ocean, taking quite a long time to do so, perhaps as much as 1,000 years. Just how this happens is complex, and is a source of uncertainty for longer range climate projections.

Finally, mathematical models of climate-like systems can exhibit sudden, unpredictable shifts. We don’t know for sure whether our climate is an example of such a system, but there is evidence encoded in ice cores from Greenland that ice age climates can jump rather quickly from one state to another. This evidence, together with our models, puts mathematical teeth on the idea of tipping points—sudden and largely unpredictable shifts in the climate state. This idea keeps many a climate scientist awake at night.

Figure 12 The effort we make to curtail emissions today will impact greenhouse gas concentrations for the rest of the century.

06c Dealing with uncertainty

As the Danish physicist Niels Bohr once remarked, “Prediction is very difficult, especially about the future.” Scientists have developed a number of different strategies to account for uncertainty, and have used these to estimate the range of possible temperatures that we will see in the coming century. There are roughly 40 climate models run by different organizations around the world, and they all give somewhat different predictions about the response of climate to increasing concentrations of greenhouse gases. In addition, we have to estimate just how the greenhouse gas content of the atmosphere will evolve over the coming centuries, which requires not just an understanding of the physics, chemistry, and biology controlling these gases but an assessment of human behavior—how much greenhouse gas will we end up emitting?

Figure 13 If nothing is done to curb emissions global mean temperature may rise by between 4°C and 13°C by 2300.

06d What the models say

The news isn’t good. If nothing is done to curb emissions, and economic growth proceeds rapidly in the developing world, by 2100, global mean temperature may rise between 2.5 and 4.5°C (that’s 4.5 to 8°F), and by 2300, by between 4 and 13°C (that’s 7 and 23°F).

The pink curve in Figure 12, RCP 8.5, is a pessimistic projection that assumes no serious effort to curtail greenhouse gas emissions and robust economic growth. In this projection, by the end of the century, the CO2 equivalent has quadrupled from preindustrial levels, to around 1,230 ppm. Paleoclimate proxies suggest that such a value has not been seen since at least the Eocene period, roughly 50 million years ago, when alligators roamed Greenland, and sea level was 70 meters (about 230 feet) higher than today’s. If the climate were to equilibrate to the associated radiative forcing of 8.5 watts per meter squared, extrapolation of the IPCC temperature projections would yield a global warming of 3–9°C (5–16°F).

The other three RCPs assume some level of mitigation of greenhouse gas emissions and are useful for estimating how various mitigation strategies might ameliorate climate change.

Unless humanity makes substantial changes in emissions or their capture, by the end of the century we could see CO2 concentrations not seen for 50 million years.

This was when alligators roamed Greenland and sea levels were 230 feet higher than today.

The projected response of global mean surface temperature depends on both the emissions trajectory and the climate model used to make the projection. In its Fifth Assessment Report, the IPCC summarizes this response, shown in Figure 13, which extends to the year 2300. The color shading for each curve in the figure represents the scatter among the various climate models used to make the projections. Note that if nothing is done to curb emissions, and economic growth proceeds rapidly in the developing world, global mean temperature may rise by between 2.5 and 4.5°C (4.5 to 8°F) by 2100, and by between 4 and 13°C (7 and 23°F) by 2300.

But what are the consequences of these changes? How will they affect us in human and economic terms? We next consider the set of real risks that climate change poses and how, at least for some risks, we might go about attaching actual numbers.

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