Climate Change: Causes and Consequences

Thank you, everyone, for coming and participating in this important conversation, and to our funders and to Beth Jacob for putting on this event. It’s really a profoundly gratifying experience to be here. I’m going to share, in my presentation, the scientific grounding that will invite us into a conversation about Jewish responses to an engagement with climate change. In this presentation, I’m going to violate some of my own rules as a teacher, which is to say, among other things, it’s going to be fairly directional and non-participatory. And also, I’m going to move pretty quickly through some technical information and show some data-driven graphs and displays that I won’t fully give us time to engage with. This is in part to provide this information in a way that is accessible to people with different underlying levels of awareness. 

A way I like to frame thinking about climate change from a scientific perspective is that, as a species, we are entering a period of time in which we are already experiencing, and will soon experience, a climate that humans have never really known. And what does this mean? So, depending on how you define human beings as a species, we have evolved over the last 20,000 to 200,000 years. This is a time enclosed within the Pleistocene period of the Earth’s past, a period in which the Earth went through a series of relatively frequent ice ages and warmer periods in between ice ages. But this is just a tiny, tiny fraction of our earth’s history, which is about 4.6 billion years old, and life’s history on Earth, which is about 3 billion years old.

And some modelers – the authors of this paper that I’m drawing from and have presented on the slide – did something, I think, really powerful in terms of our understanding of climate change, which is that they took some of the projections of how our climate is likely to change over the next 200 years or so due to human impacts, and they went back in Earth’s history and thought about the last time that our climate was relatively like the one that we are heading into. And for the business-as-usual, or sort of “we don’t do anything about the climate change” scenario, this took us back to the Eocene, which began around 50 million years ago. And during this time, the earth was quite a bit different than it is now. I’m showing here a sort of representation of what animal and plant life might have looked like in the Eocene, with some artistic license, as well as a map. 

 

On this map, areas that are uncolored, that are sort of gray, which is none of the Earth’s land surface, have a similar average annual temperature to the one we know today. And the deeper red a part of this map is, the hotter it is relative to current temperatures. So, as you can see, during this period – 50 million years ago – the Earth was so hot that ice did not form on the poles, so there were certainly no refrigerators or freezers, and there was no ice. And significant portions of the Earth’s land surface, including places where many people today live, were underwater. This is what we are heading into if we do not address climate change rapidly – within the next several hundred years. And though the Earth’s ecosystems and plants and animals – or rather, their predecessors – have experienced this kind of climate, it will be a completely new reality for human beings if we choose to go down this pathway. 

So with that as some kind of framing and motivation, I’m going to, in a very cursory way, move through what makes sense to me as a way of thinking about the science of climate change, which is to say we’ll start thinking about some of the causes of climate change, the Greenhouse Effect, the greenhouse gases that cause it, and this idea of radiative forcing that sort of brings together those themes. I’ll then think briefly and share briefly about the near future in a changing climate, and then talk a little bit about the physical and the biological or ecological consequences of climate change. 

Gases in the Atmosphere

So one thing I’d really love for you to take away, if you remember very little from this presentation, is that Earth’s atmosphere, which provides us a livable climate, is essentially like a blanket. This is where we get the idea of a Greenhouse Effect. Energy coming from the sun powers life and all of the physical processes that occur on Earth. And when that light hits our atmosphere, roughly speaking, about half of it gets deflected off. The remainder enters the Earth’s atmosphere and either sort of participates in or interacts with our atmosphere there, or enters the Earth’s kind of surface system. Some of that energy that enters the Earth’s surface system is radiated back out into space, and some stays inside the Earth system, meaning inside our atmosphere.

That energy that is retained makes life on Earth possible. Changing it also alters how warm it is on Earth and how much energy there is powering stuff on our planet. By thickening our “blanket” – our atmosphere, which covers the Earth – we keep more energy inside. And that is a process that has happened and reversed many times during Earth’s history. And right now, by emitting certain gases into our atmosphere, we are thickening the blanket, or making the atmosphere better at retaining heat. 

Not all gases that we release into the atmosphere, or take from it, have this effect. This gives us the idea of greenhouse gases – or gases that do contribute to the ways in which the Earth’s atmosphere retain heat. And so I’ve thought a lot over the years about how best to engage with this idea. And what I’m going to do in this presentation is present to you four greenhouse gases, and talk about how they operate within our Earth system. These are kind of the four “big ones” to a large extent, and they’re also representative examples of other greenhouse gases that I’m going to allude to? in my talk. 

I just want to note, because I think this is a very important fact as a scientist, that 78% of our atmosphere is nitrogen gas. This is the bulk of what we breathe in and breathe out – it is not a greenhouse gas. It does not interact with energy entering or leaving our Earth system. The same goes for oxygen – a very important gas to us, but completely irrelevant to climate change in a direct sense. So the bulk of the gases that make up our atmosphere are sort of inert or irrelevant in this context. It’s really a few trace gases that have the biggest effect.

Here are the big ones, and I’m representing them in English-language names, chemical formulas, and structural formulas. For those of you who, like me, are dorks, I’m not going to go into these, but I’m happy to chat about them later. The big greenhouse gases are:

• Carbon dioxide – this is sort of the number one, for reasons that we’ll cover shortly. And our earliest, and sort of most compelling evidence for, the link between changes in the concentration of this gas over time and climate change come from observations of CO₂ concentrations in the atmosphere from the middle of the century forward. 
• Methane, which also has carbon as a central element in its structure – so, it shares that with CO₂.
• Nitrous Oxide, which is nitrogen-based. 
• And then this one is kind of just a paradigmatic example, instead of one of the absolute major ones. Sulfur hexafluoride is one of a series of fluorinated gases, meaning they have lots of fluorines, or Fs, in their structure. These are produced entirely by human-mediated industrial and commercial processes. 

So these are the four big ones. I think if you understand a little bit about the atmospheric chemistry of these four gases, you will understand a lot about what a greenhouse gas is, and furthermore, how they impact the climate. And that is through a process called radiative forcing. So this kind of obscure term basically means the difference between the sunlight absorbed by the earth’s surface and the energy radiated back out into space. So as it gets bigger, a given gas, or a given group of gases, is keeping more energy inside the earth. And as it gets smaller, or even negative, those gases are not engaging with energy, or are cooling the earth. 

There are three factors that affect the radiative forcing potential of greenhouse gases in our atmosphere, and we’re going to learn about them right now. And I’m putting the greenhouse gases here just as an example. The first one is gas-specific. This is maybe the most intuitive. In some ways, different gases are either better or worse at keeping energy inside the earth’s system. Carbon dioxide is very powerful, but it is not the most powerful greenhouse gas in terms of its specific global warming potential. Methane, for example, the second carbon-based gas, has a capacity over an order of magnitude higher to keep energy inside the Earth system. The fluorinated gases have a really large capacity to do so. And I’m not a chemist, so my knowledge kind of stops here. But this has to do with the type and the orientation of the chemical bonds between the atoms that make up these different molecules. So this is sort of related to what the gas is – its identity.

The second one is related to the residence time of each gas. This is the average amount of time in years that a gas stays in our earth’s atmosphere – that a given molecule does. So if you breathe out a molecule of CO₂, it will – on average, average it across all the CO₂ molecules in the entire Earth system – stay in the atmosphere for four years. If you breathe it out next to a tree, it’ll probably spend only a few minutes in the atmosphere and get sucked up by that tree’s leaves. For instance, CO₂ has a residence time in the atmosphere of about four years. Methane’s is twice as long. Nitrous oxide is an order of magnitude higher. And unfortunately, the fluorinated gases, without some other intervention, stay in the atmosphere for thousands of years. 

Finally, and the kind of last piece of the puzzle, is the bulk amount of each of these gases that gets emitted into the atmosphere. And I chose just, I thought, a comprehensible graph of U.S. greenhouse gas emissions for 2020. This is representative of broader trends. We, as human beings, emit a lot of CO₂ into the atmosphere, less methane, less nitrous oxide, and then the least amount of fluorinated gases. 

So, taken together, these three factors lead to the kind of gross radiative forcing for a particular gas, because we emit a lot of it, and it’s relatively good at keeping energy inside the earth system. CO₂, carbon dioxide, is our kind of most important, and most talked-about, greenhouse gas. Methane is secondary in its effects. Halogenated gases are third or fourth, depending on how you slice things. And nitrous oxide is perhaps the fourth of these groups.

Okay, I’m going to leave this so we can keep moving and think about what climate change means for us as people living on Earth. But this is now a crash course that you’ve been through about the global atmospheric chemistry and physics behind climate change. 

What Will Climate Change Feel Like?

And we’re also moving more into my area of expertise. This always makes me nervous when I know there are chemists in the audience. The science of climate change projection or thinking about what our climate will be like in the future, is also an incredibly complex and computational one that I do not participate in on a professional level. 

So I’m going to give you the sort of “how Jake sees it” view of this. I think it’s important to understand a little bit more about Earth’s climate history before diving into what is about to happen. And I actually really love this figure on Wikipedia that I’m just adapting shamelessly. It does a really good job of explaining some kind of complicated things.

So we have some ideas about what Earth’s climate history was like from the Earth’s formation through to 500 million years ago. So – the first eight-ninths of Earth’s existence, we actually don’t know a lot about it, because the evidence of what the climate was like tends to be erased by things happening on Earth. We have a better sense of what climate has been like for the last half a billion years, and that’s represented here on this slide.

What I will draw your attention to is that around the Eocene, so around 50 to 60 million years ago – this is like dinosaur and tree-fern time – the earth was really, really hot. We call the period around 55 million years ago when it was at its hottest. Most recently, the PETM, or Paleocene Eocene thermal maximum, the last, hottest time on Earth with a lot of fluctuation. Ever since that point, the Earth has gotten cooler. And I’ll note here that this line, this horizontal line that runs through the whole chart, represents a global temperature like the one that we all collectively know. So points or parts of the line that are above, that are warmer than what we know and points that are below the line are cooler than that. 

Over the last million years or so, the earth has been characterized by these relatively stable cycles of cold and warm, those glacial interglacial cycles I mentioned before. And about 20,000 years ago, during the Pleistocene, the Earth was the coldest it’s been most recently. We call this the last glacial maximum. This is a period of time in which, for instance, much of northern North America was covered in glaciers. 

Since the last glacial maximum, over the period of time that human civilization as we know it has developed, the Earth has been getting gradually a bit warmer. This is a pattern that human activity has had nothing to do with. Our impacts kind of begin having a major effect, our impacts become significant, right here at the end of the graph, only over the last 200 or so, and really 150 and really, really the last 70 or so years, as economic activity, change of the way we use land, implementation of new technologies, have caused us to produce many more of those greenhouse gases. 

So now that we know a little bit about the Earth’s climate history, and I could go over so easily, I just want to say this. So as a plant biologist, something I think about that helps me make sense of all of this is that all of the plants that we know today, of course, are in the process of dynamic evolution and diversification and loss of diversity. If we traveled back in time 30, 40, 50 million years ago, we would not see the plant species we know today because they hadn’t evolved yet, but we would see plants that kind of looked like them, or were reminiscent of them. Probably the same is true for many animals. Going further back, we would not see that. So I just want to stress that I think it’s interesting, as a biologist, that the evolution of much of the life that we know and depend on, not just us, happened during this somewhat unusual period of time in which the Earth’s climate was really hot and then got cooler. That’s just interesting to think about. 

The UN-sponsored international panel that does climate science, the IPCC, has fairly recently, in its last cycle, adopted a new form of thinking about climate change projections. This is the idea of shared socioeconomic pathways, or SSPs. Essentially, the number in these different scenarios that tell us how our climate might change tells us the number of degrees of warming that we will have committed to by 2100. So this fossil fuel development, SSP 8.5, means that by the year 2100, our climate will, on average, be about 4 to 5º warmer than it is now. But we will have emitted enough greenhouse gases to bring us to eight and a half degrees of warming, which would be truly catastrophic, by, let’s say, 2200 or 2250. 

The sustainable scenarios, the ones that are probably an option for us right now, entail only one to two degrees of warming by the end of this century, and we would probably stabilize around those levels going forward into the future. So these are some of our options, given the best science we have right now, and pending, of course, a lot of contrasting approaches to managing emissions to the atmosphere.

Consequences of, and Responses to, Climate Change

Okay, in my remaining time, I’m now going to switch over to what I tend to think about more, which are some of the responses to climate change. And again, this is a huge body of thought that I’m condensing down into a few bullet points. So what I’d love for you to take away, and what many of you are probably already familiar with, are just a few – let’s say – five physical consequences of all of this gas that we’re putting into the atmosphere. 

I’ll start with the most obvious one. Climate change is often called “global warming,” because we now have evidence that the earth’s climate is getting warmer as a result of the emissions that we are releasing into the atmosphere. You can find a bazillion graphs that show this in different ways. I like this one that has, again, average temperature over the last century. Blue years are years in which the temperature was colder than that average, red are above. We’ve been in the red since the 1980s. 

I want to emphasize that this pattern is a result of greenhouse gas emissions, and the amount of emissions we add to the atmosphere will have an effect on this trend. So, on the left, in the set of graphs, you can see a variety of scenarios in which we do not, in red, or do, in the other colors, essentially flatten the curve, to use a COVID metaphor, and reduce our emissions. This will lead to a lag in warming, but will lead to warming over, if not our lifetimes, the lifetimes of our children and grandchildren. 

A second – and to me, really critical, as a plant person – consequence of climate change will be alterations in the hydrology or the patterns of rain and snow that fall on our earth’s surface. And this will be highly complex and will vary over the course of a year and over space, depending on lots of other factors. One way to think about this is that as we trap more heat or energy inside our earth system, we are essentially super-powering the systems of storms and rain and snow that accumulate across the earth’s surface. So for North America, where we’re all living right now, and for northern North America, where we’re located, this is, under some scenarios, likely to lead to wetter winters and springs and perhaps drier summers. 

And you might say, “Well, the extra precipitation during the winter and spring would make up for the drought during the summer.” That’s not very helpful if you’re a plant that needs water in your roots during the growing season. So, as you can see from this map, even within a single continent, these patterns are very spatially variable.

A third, and somewhat challenging-to-think-about, physical consequence of climate change, but one that is significant – because it then has the capacity to feedback into ongoing climate change – is something called albedo. So, albedo is a measurement of how much energy hitting the Earth’s surface is reflected back out into space. It varies from 0 to 1. It’s a proportion. Zero would indicate total absorption of all energy, like the blackest black color you can possibly imagine, that just immediately absorbs all energy. And then 1 would be a totally reflective white surface that sends all energy back into space. As albedo gets lower, we make climate change worse. And as albedo gets higher, we cool the planet, essentially.

And there is a lot of concern that because climate change is causing glaciers and permanently ice-covered areas to melt, we are reducing albedo and leading to positive feedback to climate change. And this is probably happening in some areas. But I want to emphasize that in this respect, as in many others, climate change interacts with other global change drivers, like urbanization. So at the same time that we’re melting glaciers, we’re also converting a lot of dark-colored natural landscape into light-colored cities, which actually increases albedo. So as much as I am often just a huge downer about climate change science, this actually is one area where we may be inadvertently making some of our problems less bad through creating other problems. 

A very well-known and not inconsequential impact of climate change is that as we melt these glaciers and this ice, we are also causing sea levels to rise. There’s not much else to say here other than that I want to emphasize that only about two-thirds of sea level rise stems from the addition of melted snow and ice. The other third (and maybe I just think this is cool physically), results from the thermal expansion of water. So water, like other substances, takes up more space the warmer it gets. That’s just a fun cocktail party fact. I tell my students cocktail party facts all the time, and I don’t know if any of them, like, go to cocktail parties, but they’re going to be full of them when they do.

I would be remiss as a terrestrial person if I didn’t at least briefly mention this final physical consequence of our greenhouse gas emissions, rather than of climate change itself. So as we add more carbon dioxide to the atmosphere, that pushes more carbon dioxide into our world’s oceans, which are becoming, because of that, slightly more acidic, through a chain of events that I’m happy to walk you through, not in this presentation, right now, in the interest of time. This makes it harder for marine invertebrates, like clams, to form their shells, which are made of calcium carbonate. It’s not that the acid melts their shells. It turns into another ion called bicarbonate in water that then kind of steals the carbonate or chalk away from these organisms that they need to kind of build their bodies. This is significant for a lot of different reasons, but I’m not going to get into it right now. 

Organisms Adapt

In my last few minutes, I’m going to focus on at least my true interest and love, which is some of the biological or ecological consequences of climate change. There are a lot of different ways to think about this. The way I kind of divide it up in my mind is the four responses that organisms can have to climate change, and I’m going to go through them in order of severity or sort of from the most severe and short-term to the maybe least severe and the longest-term. So these are mortality, migration, acclimation and adaptation. 

Tragically, and as a source of much grief, climate change is likely to lead us to what has been called the Sixth Extinction, a massive loss of biodiversity across the earth’s surface. In some of the best modeling we have of this, including this 2020 paper in the Proceedings of the National Academy, it’s estimated that, for instance, for tropical plants, under a business-as-usual or extremely high emission scenario – shown as the light green color on the outer ring of this chart – in excess of 30% of tropical plant diversity could be lost. This is the result, perhaps needless to say, of climate conditions changing faster than organisms can escape, adjust or evolve in response to them. 

So what happens when organisms are able to save themselves, and don’t die, and then become extinct globally due to climate change? Migration, or movement in space, is one option. And this image from the Nature Conservancy shows likely migration pathways for mammal, bird and amphibian species under climate change scenarios. As you can see, these reveal a lot of really fascinating patterns, including a sort of migration superhighway from the Southeastern United States, which is quite biodiverse, through the Appalachian Mountains, up into northeastern Canada. I’ll just note, too, that plants are able to migrate as well – by sending their seeds out, and having differential survival of seeds in cooler or higher altitude places. But this is a kind of slow process, especially for long-lived plants.

If death is avoidable and migration is not an option, a third pattern of climate change response is what I study, and this is acclimation. And I’ll just very briefly include a little bit of my own research. This is within-generational, within-individual response to climate change. So, this is not related to evolution. Biologists tend to distinguish between acclimation and the next thing I’m going to talk about in this way. So, this is one individual or one group of individuals becoming more resilient to climate change. 

In my lab, we study stress that occurs over the course of a single year as a proxy for climate change stress. My students collect samples from plants out in the field at different times of the year, when they’re experiencing different amounts of stress, and then bring them into the lab and torture them to understand what the consequences of those types of stress are. For instance, we study turgor loss point as a metric of drought vulnerability. This is the level of hydration at which a given plant will sort of wilt and begin dying. We collect field-sampled leaves and rehydrate them in the lab. We then measure some things about their chemical contents and are able to extrapolate a turgor loss point or a point of kind of hydraulic no return for plants. And when we did this for 27 species from across the plant tree of life, collected from arboreta in Pennsylvania and Massachusetts, we were able to look at patterns of plant resilience to climate change over a single year. 

And so, in the series of graphs I’m going to show you, the points that are lower down in these graphs show more resilience, and plants that are higher up show more vulnerability to drought. In this case, three-quarters of the species we surveyed showed a predictable pattern of increased resilience to drought over a single growing season. And to wave my hands a little bit, to me, this is evidence that our changing climate, though it will lead to mortality for many plant species, actually may also come up against plants’ and other organisms’ capacity to acclimate. So this is, in some ways, a hopeful message that we may have a set of species that populate our forests right now that are able to respond to climate change, at least to a certain extent. 

Finally, I just want to highlight one more research story to show the fourth, and maybe most long-term, response to climate change ecologically, which is adaptation or evolution. So this, for biologists, means change transgenerationally – from parent to offspring, over many generations – and occurs at the level of populations rather than individuals. Dr. Morgan Kelly’s research on intertidal copepods – these are small microscopic animals that live in water – showed this really cool pattern where across the west coast of the US and Canada in the Pacific Ocean. Copepods that lived in warmer areas were able to tolerate warmer conditions, which makes sense. But she also found that over time, when exposed to warmer conditions in the lab, the populations of copepods that were most vulnerable were often best able to adapt or evolve, over only a few generations, to a warming climate. So this, perhaps, provides one vignette, or an example, of how ecological communities may evolve if given the opportunity to do so to climate change.

Conclusion and Takeaways

I realize I’ve gone over time, and in some parting thoughts, I just want to share these take-homes that I hope you will take away with you. 

The first is that the future is unwritten. So we’ve already created a substantial amount of climate change, but how much we create more is up to us. 

Second, nature is resilient. So, as the research my students and I do shows, we often think of nature as being very fragile and as something that humans are harming, but that is not – I think that narrative needs to coexist along with an understanding of nature’s resilience. 

And finally, rapid, widespread change is possible. These are us greenhouse gas emissions by year, and – I need to change that – blue should be 2019. I wanted to illustrate that various events in our country’s politics, economy and epidemiology have caused changes in greenhouse gas emissions – for instance, declines starting in 2008 with the implementation of a variety of approaches to reducing greenhouse gas emissions, and then again in 2019, when our country was responding to another existential threat of COVID. So I want to emphasize the possibility of change. 

And with that, I’ll thank you very much for your attention and invite any other questions either during our question and answer session or to me personally in some other venue. Thank you all very much.