Climate

Summary of Climate Overview

This section focuses on understanding climate, defined as the statistical description of weather patterns over time, including mean and variability of surface variables like temperature, precipitation, and wind.

Key Concepts

Climate Definition: According to the IPCC, climate is measured over periods from months to millions of years, encompassing both averages and variations.
Koppen Climate Classification: This system categorizes the world into five climate regions: humid tropics, arid regions, humid mid-latitudes, continental regions, and cold polar regions.
Climate Change: Refers to the variation in global or regional climates over time, driven by internal atmospheric forces and external factors like solar activity and human activities.
Temperature Changes: Over the last 50 years, significant warming has been observed, particularly in the Arctic and northern hemisphere, with few regions experiencing slight cooling.
Climate Models: These are used to simulate interactions within the climate system. Models incorporating human activities (like fossil fuel emissions) show a clear increase in global temperatures, aligning with observed data.
Climate Variability: This includes both random variability (noise) and periodic variability (cyclical patterns). Understanding these variations is crucial for predicting extreme weather events.
Climate Engineering: Also known as geoengineering, this involves intentional modifications to the climate, such as reflecting sunlight to cool the Earth. However, these methods carry uncertainties and potential side effects that require further research.

Conclusion

The study of climate encompasses its definition, changes, modeling, variability, and the potential for engineering interventions. Human activities, particularly greenhouse gas emissions, are significant contributors to climate change, and while engineering solutions exist, they come with risks that need careful consideration.

(upbeat music) - This section is our focus on climate. Climate is an atmospheric pattern. And we're gonna think about its definition how climate changes, how we model climate, the variability in climate, and even think a little bit about how to engineer climate. And this is part of our learning objective of describing the atmospheric composition and structure including to think about how it's changing. The IPCC definition of climate is that it's the statistical description and state of the weather system. And so it's measured in terms of the mean and variability of relevant quantities over a period ranging from months to millions of years. These quantities are the most often surface variables such as temperature, precipitation, and wind. And these are the quantities that we think about most often as associated with climate. Climate measures more than just averages. So we said mean and variability. So what does that mean? It means that it also includes the magnitudes of day-to-day or year to year variations. And so it's not just the average, it's how much it changes between years and between days. Here's one way to classify climate called the Koppen Climate Classification. And this is a great classification because it does exactly what you would think it should. It splits the world into regions where there's humid tropics, arid regions, human mid-latitudes, other continental regions and cold polar regions. And so these five categories are kind of what you'd think about, right? In terms of arid deserts like out to the east of San Diego. And then we have humid regions like on the coast of California. If you go to the tropics, you're gonna have much warmer types of climate and then in the polar region it's going to be colder. The variation in global or regional climates over time is what we call climate change. And this is caused by both internal forces and external forces. What do we mean by that? So internal are things that are part of the atmosphere and the climate system. And external forces are things like either the sun that's outside of the earth system or humans that are having effects in addition to what the biosphere would otherwise have. So temperature change in the last 50 years is one way to think about climate change. And this illustration shows us the approximate change over the last 50 years. And so what's done here is to first take an average from 1956 to 1976, so that 20 year average and compare that to the last 10 years from 2011 to 2021. And the difference is shown at the bottom here in Celsius and Fahrenheit, and what you can see is that the biggest changes in temperature over the last 50 years have been in the Arctic region as well as to a lesser extent in the Antarctic region. There's also been a lot of heating between one and two degrees, over much of the northern hemisphere continents. There are a very few regions where it's actually gotten slightly colder. And these are mostly in the southern ocean, in the southern hemisphere. And so this is the change in climate where the average temperature has increased over large regions over a 50 year period. And that's what we mean by climate change. So climate models are how we try and understand why this change is happening. And what models do is they use quantitative methods to simulate the interactions of the atmosphere, oceans, land, surfaces and ice. And a simple illustration is this graph at the right. You may have seen this other places before. Here we have global surface temperature since 1850 through 2020. On the left in Celsius on the right in Fahrenheit relative to that 1850 temperature. So zero means the temperature of 1850 and this is showing the relative change. So what you can see is if we put in the model only natural drivers in green, then the temperature stays about flat. It stays about no change from 1850. But if we put in the model humans burning fossil fuel to emit carbon dioxide and methane and other greenhouse gases, then you get a large increase over the last 40 to 60 years. And that's shown in the red line. And so this difference between the red and the green, is the effect of humans. On the other hand, we can look at what's actually been observed and that's in black. So the observed temperature clearly matches the model with the red where human and natural causes are included. And so from that we know that it's the humans and in particular carbon dioxide and methane emissions giving us greenhouse warming. So climate variability is an important term to describe the variations in this mean state that we've been thinking about the change in mean temperature. So that's our average change. But of course we're interested not just in the average but also in the minimum and the maximum. And those are what we often think of as the extremes and include extreme weather. But here we have the variability and we can think of variability in two different ways 'cause there's two different types. There's random variability and this is what you might just call noise. And it includes noise from sound, it's inherent to the system and it's randomly distributed in time. And so if you see enough variability, that doesn't really change, its min and max over time, that means it's random variability. And on the other hand if you see the men and max change over time, that's what we might call periodic variability because it has a cyclical behavior where it goes up to a maximum and down to a minimum and up to a maximum and down to a minimum. So there's distinct patterns in this periodic variability that are regularly repeated. And sometimes we call these an oscillation or a cycle instead. And we can we'll think about some of these cycles as part of this course. The other important thing about climate, is to think about climate engineering. So climate engineering is what we might call intentional climate modification. And in thinking about this and it's sometimes called geoengineering, what we're thinking about is ways that we could actually intentionally change the climate. Since we've succeeded in changing the climate unintentionally by emitting carbon dioxide and methane. Maybe there's something that we could do that would fix that problem or even make it better. And so one idea has been to think about such deliberate and large scale interventions as putting particles in the air to reflect sunlight and reflecting sunlight as we'll see later in the course will make the earth colder. And so this would be a couple different methods here to think about. One is putting aerosols in the stratosphere and the other is adding particles to clouds just like our current pollution sources do. And both of those things make sunlight reflect more back into space and give a cooling effect. Unfortunately, there are large uncertainties with both of these approaches and potential side effects as well as unforeseen consequences. So a lot of research would be needed, in order to show whether this could help or might in fact hurt the situation of global warming. So to sum up, we've thought about climate in terms of our learning objective and how it is one of the patterns of the atmosphere and contributes to its structure. It's defined to be this statistical change in the patterns of weather over time. It is currently changing and we have shown with models that it changes because of human influences from carbon dioxide emissions as well as methane. And then it has variability. It doesn't mean it's the same all the time it means it changes with periodic as well as random variability. And we can think about trying to engineer the climate but that has a lot of uncertainties. So that wraps up our focus on climate.