[VIDEO] Climate Conversations: What is behind the record-breaking fall extremes?

by
Climate Signals

Speakers

Noah Diffenbaugh, geoscientist and Professor of Earth System Science at Stanford University

Mike Mann, professor at Penn State University and director of the Earth System Science Center


Summary

The fall season in 2019 is shaping up to be a wacky one in the U.S. We've seen some early snow fall and record cold in the Rockies. Heat records broken across the East, extreme winds in the West. Now we're preparing for a cold front, that could bring snow as far south as Tennessee. But are these extremes that connected? My name is Markeya Thomas, and we have Dr. Noah Diffenbaugh and Dr. Michael Mann here to help explain the weather extremes we're seeing. 

Click here to jump to the full transcript.


Highlights

On the Arctic and jet stream influence on mid-latitude weather

The unusual warming of the Arctic appears to be influencing the behavior of the northern hemisphere jet stream in a way that gives us those large undulations where you see big peaks and troughs in the jet stream. These changes in the jet stream are leading to increased weather extremes of various types, including increases in some extreme winter events.

Michael Mann

One pathway by which global warming is influencing extremes is through dynamic effects, changes having to do with the effects of forces on the motion of objects. We’re seeing an increase in the frequency of ridging patterns—like the one we’re seeing right now—that are creating warm conditions in the West and colder conditions in the East. My research has identified the pattern of warming in the lower and middle atmosphere as a primary driver of that change in the atmospheric pressure patterns.

Noah Diffenbaugh

We're seeing that monumental changes in the Arctic—the warming of the Arctic and the loss of the sea ice in the Arctic—are changing the patterns of the northern hemisphere jet stream in a way that gives us the various types of persistent weather extremes we've been experiencing in the summer, in the fall and in the winter. All of this is tied together and if we tend to underpredict one change, there's a good chance that we're going to underpredict other changes because of the interconnectedness of the system.

Michael Mann

My research group has worked on the juxtaposition of warm conditions in the West and cold conditions in the East during winter. That's essentially the atmospheric set up that we're experiencing now in the U.S. The simultaneous occurrence of warm conditions in the West and cold conditions in the East has been increasing over the last several decades here in the U.S.

Noah Diffenbaugh


On extreme winter weather

Temperature contrasts are what drive typical mid-latitude storms. Emerging science based on high resolution modeling, looking at the characteristics of these storms, suggest that the warming of the oceans and climate change is increasing the intensity of nor'easters.

Michael Mann

Temperature contrasts are what drive typical mid-latitude storms. So you've got these large temperature contrasts, which contribute to the strengthening of the storm, and you've got what we call latent heat released from the ocean that's contributing to the energy of the storm. And it comes together to give us these fascinating systems called nor'easters.

Michael Mann


On climate change and wildfires

The fact that we're seeing unprecedented wildfires right now in the subtropics of the northern hemisphere—California—and the subtropics of the southern hemisphere—in New South Wales, Australia, and Queensland, Australia—that is symptomatic of a pretty robust prediction that those areas get drier and hotter. And when you get drier and hotter conditions, well, you get wildfires.

Michael Mann

The conditions on the ground when those strong winds occur are really critical for wildfire risk. In recent years in California we've had very warm conditions and very late starts to the wet season. What that has meant is there are record dry vegetation conditions when these autumn winds blow.

Noah Diffenbaugh

We're seeing elevated risk coming from the added heat in the climate system that's causing there to be higher temperatures in the summer and autumn, and greater aridity of the vegetation. When that dry vegetation is combined with very strong winds, that creates a very high wildfire risk and very extreme wildfire behavior like we've been seeing in California in recent years.

Noah Diffenbaugh

You take extreme heat, you take extreme drought, you put them together, you get wildfire. This isn't rocket science. What we're seeing now here in the United States and what we're seeing simultaneously in Australia right now is symptomatic of these changes that we predicted decades ago. Our predictions are coming true, unfortunately.

Michael Mann


On conservative model predictions, uncertainty, and risk

The dynamical mechanisms that relate to the behavior of the jet stream are at the forefront of current climate research and some of the mechanisms that we think are important here don't appear to be well captured in many of the climate models. So this is a reason for caution. We may actually be underestimating the impact of climate change on some of these extreme weather events because of these more subtle connections that have to do with the jet stream where we're still trying to understand them and they're still not perfectly represented in the climate models.

Michael Mann

Arctic sea ice decline is a very good example of where the predictions have in some sense been fairly conservative. We've seen Arctic sea ice decline at a rate that exceeds what the climate models tended to predict and as our understanding improves, we recognize that there are certain processes that are relevant to understanding the way that sea ice reacts to changing wind patterns that weren't well captured in the climate models. As climate scientists start to try to account for these processes, we're doing a better job, but still the observations appear to be ahead of the models. 

Michael Mann


Full transcript

MARKEYA

The fall season is shaping up to be a wacky one. We've seen some early snow fall and record cold in the Rockies. Heat records broken across the East, extreme winds in the West. Now we're preparing for a cold front, that could bring snow as far south as Tennessee. But are these extremes that connected? We have Dr. Noah Diffenbaugh and Dr. Michael Mann here to help explain the weather extremes we're seeing.

So first question, October's weather set records for heat, cold, rain and snow in the U.S. Is this what climate change looks like?

MANN

Many of the unprecedented weather extremes we’ve seen in recent years have been extreme summer events—heat waves, wildfires, et cetera—but we're also seeing increases in extremes during the fall and winter and spring. 

We're seeing increases in extremes during the fall and winter and spring, and there is a signature of human-caused climate change. 

The unusual warming of the Arctic appears to be influencing the behavior of the northern hemisphere jet stream in a way that gives us those large undulations where you see big peaks and troughs in the jet stream. These changes in the jet stream are leading to increased weather extremes of various types, including increases in some extreme winter events.

DIFFENBAUGH

We can think about changes in extremes due to climate change from the perspective of at least two components. And the first component is that we are adding heat to the climate system, and so scientists call that the thermodynamic effects. This just means that the added heat in the climate system is increasing the energy that's going into heatwaves, increasing the water vapor that's in the atmosphere so that when we get these atmosphere and ocean conditions that have always occurred on earth in terms of just the weather patterns that have always occurred on earth, there's now an elevated risk of extremes from the extra heat that's been added to the climates. And we see this in heat waves, we see this in heavy rainfall events, we see this in storm surge flooding from the elevated sea level that has resulted from that added heat to the climate system. So that's one pathway by which global warming is influencing extremes. Then the second pathway, dynamic effects, is concerned with the effects of forces on the motion of objects and how those are being influenced by global warming and the associated changes in the climate system.

One pathway by which global warming is influencing extremes is through dynamic effects, changes having to do with the effects of forces on the motion of objects. We’re seeing an increase in the frequency of ridging patterns—like the one we’re seeing right now—that are creating warm conditions in the West and colder conditions in the East. My research has identified the pattern of warming in the lower and middle atmosphere as a primary driver of that change in the atmospheric pressure patterns.

MANN

Some of the changes due to global warming are pretty basic, right? You warm up the planet, you're going to have more frequent and extreme heat waves. That's a given. You're going to put more moisture in the atmosphere so you can get larger rainfall events, worse flooding events. You're going to bake the ground, you get worse continental drought; you take that heat, you put it together with drought, you get wildfire. So some of this is fairly basic and the mechanisms are captured well by the climate models that we use both to study the present and past and to project into the future.

The dynamical mechanisms that relate to the behavior of the jet stream are at the forefront of current climate research and some of the mechanisms that we think are important here don't appear to be well captured in many of the climate models. So this is a reason for caution. We may actually be underestimating the impact of climate change on some of these extreme weather events because of these more subtle connections that have to do with the jet stream where we're still trying to understand them and they're still not perfectly represented in the climate models.

There are certain very robust features of climate simulations that have stood the test of time, decades ago. Even the crude climate models that we were using at that time predicted the so-called Hadley cell circulation, which is when you have rising motion near the equator and sinking motion in the subtropics. With the sinking motion you get dry hot conditions. Well, the models predicted that human-caused warming would actually cause a migration of that descending branch of the Hadley circulation. The belt of deserts moves further into the mid-latitudes, and so the fact that we're seeing unprecedented wildfires right now in the subtropics of the northern hemisphere—California—and the subtropics of the southern hemisphere—in New South Wales, Australia, and Queensland, Australia—that is symptomatic of a pretty robust prediction that those areas get drier and hotter. And when you get drier and hotter conditions, well, you get wildfires.

MARKEYA

Thank you. Another major record set in October was Arctic sea ice extent. It reached its lowest point on record for the month. Surface temperatures in the Arctic have been way above average. I'm wondering if the record heat and melt in the Arctic is connected to record cold that we're seeing in the United States?

DIFFENBAUGH

First and foremost, it's important to emphasize that the loss of Arctic sea ice is one of the most profound changes that we're seeing in the climate system, and when you analyze the statistics of the loss of Arctic sea ice, it's just phenomenal. I mean, statistically phenomenal. It's essentially impossible from a statistical perspective to have the level of decline that's occurred without this longterm forcing from human-caused climate change.

We know with very high confidence that the heat that is being added to the climate system through human emission of greenhouse gases is causing a very rapid decline in Arctic sea ice. It is statistically a new normal, in terms of the range of summer sea ice levels, and now we're seeing this in autumn sea ice levels as well. I bet that new range is completely removed from the historical range just over the last four decades, we're now seeing that even the highest sea ice years in the Arctic are lower than what used to be the extreme low. And we're really approaching the prospect, the very real prospect of the Arctic being seasonally ice-free within the next few decades, which is something that scientists had predicted.

MANN

Arctic sea ice decline is a very good example of where the predictions have in some sense been fairly conservative. We've seen Arctic sea ice decline at a rate that exceeds what the climate models tended to predict and as our understanding improves, we recognize that there are certain processes that are relevant to understanding the way that sea ice reacts to changing wind patterns that weren't well captured in the climate models. As climate scientists start to try to account for these processes, we're doing a better job, but still the observations appear to be ahead of the models. 

We're seeing a decline in Arctic sea ice that is beyond what the models tend to predict. And that underscores a fairly widespread theme that uncertainty is not our friend. Uncertainty can cut against us and in this respect it appears to be cutting against us and there were surprises in store.

And one of those surprises are stuff that Noah's worked on, and my group has worked on. Our good friend Susan Joy Hassol likes to say that "What happens in the Arctic doesn't stay in the Arctic," and that is absolutely the case here. 

We're seeing that monumental changes in the Arctic—the warming of the Arctic and the loss of the sea ice in the Arctic—are changing the patterns of the northern hemisphere jet stream in a way that gives us the various types of persistent weather extremes we've been experiencing in the summer, in the fall and in the winter. All of this is tied together and if we tend to underpredict one change, there's a good chance that we're going to underpredict other changes because of the interconnectedness of the system.

MARKEYA

Thank you. So more generally are we saying, or trying to say that the U.S., eastern U.S. has frequently thing cooler temperatures relative to the warmth that the more that in warming observe locally? Sorry, tongue-tie today.

DIFFENBAUGH

My research group has worked on the juxtaposition of warm conditions in the West and cold conditions in the East during winter. That's essentially the atmospheric set up that we're experiencing now in the U.S. The simultaneous occurrence of warm conditions in the West and cold conditions in the East has been increasing over the last several decades here in the U.S.

In terms of the mean temperature change, the West has warmed more rapidly than the East. The East has warmed relatively gradually during the winter months, and the juxtaposition of the warm West and cold East events is associated with a very distinct atmospheric circulation pattern with high pressure anomalies over the West and low pressure anomalies over the East. 

There's not yet a statistically significant change in the frequency of occurrence of that pattern, although we do find an increase in the mean occurrence. But what's really key is that when that atmospheric circulation pattern occurs, it's much more likely to produce very warm conditions in the West combined with, with cold conditions in the East. So what we're seeing is that the primary driver is, is the resulting temperature severity that happens when that atmospheric circulation pattern sets up.

MANN

I'm going to sound a little bit like a broken record here because, and again, it comes back to this issue of uncertainties and uncertainties not really breaking in our favor—surprises that are in store that aren't welcome surprises.

I've been particularly interested in the intense nature of nor'easters. I live on the East Coast of the U.S. in the Mid-Atlantic in Pennsylvania, and we are influenced by these extremely strong mid-latitudes systems called nor'easters that have some of the characteristics of a tropical system. Like a tropical system, part of the energy that intensifies these storms comes from the heat released from the ocean when it evaporates moisture.

As we warm up the oceans and we continue to get these clashes between warm maritime conditions and the cold Arctic air masses that travel from west to east and eventually come to the Eastern U.S. and collide with the warm maritime air, those temperature contrasts are what drive typical mid-latitudes storms. So you've got these large temperature contrasts, which contribute to the strengthening of the storm, and you've got all this, what we call latent heat released from the ocean that's contributing to the energy of the storm. And it comes together to give us these fascinating systems called nor'easters.

We still don't model these perfectly. In fact, in most climate models, these sorts of storms are represented fairly crudely, just like hurricanes tend to be represented fairly crudely. Some of the emerging science based on high resolution modeling, looking at the characteristics of these storms, suggest that the warming of the oceans and climate change is increasing the intensity of these nor'easters. Now what's interesting about that is these storms spin around and when they spin around, they bring warm air from the south up on one side of the storm on the east side of the storm. But on the west side of the storm, they bring this cold Arctic air down and the stronger the storm, the more spinning, the more intense the spinning, the more intense the mixing and the more intense the mixing, the more of that cold air you draw down. It's one of my pet theories that some of the extremes that we've seen in the winter along the U.S. East Coast, and this really only applies to a particular region along the U.S. East Coast. Some of those cold extremes appear to be a product of these record strength nor'easters. And again, there's lots of uncertainty because we don't really capture these changes, these characteristics in the large-scale climate model simulations that are often done to project future climate change.

MARKEYA

That's very helpful, thank you. Speaking of things similar to hurricanes out west, particularly in California, we've seen back-to-back searches of hurricane-like winds. This is the third year in a row that these winds have worsened wildfires burning in the state. Is climate change, worsening their effects and are these tied to any other weather patterns causing extreme temperatures.

DIFFENBAUGH

This is also and area of active research and what's critical is that these winds combined with other conditions to create wildfire risk, and wildfire is a product of the physical conditions, the winds, how dry the vegetation is, how dry the atmosphere is when the winds occur, but also exposure and vulnerability and critically, ignition. More than 90% of the fires in California have a human ignition, so it's important to keep in mind that all of that comes together to create wildfire risk. The conditions on the ground when those strong winds occur are really critical for the wildfire risk. In recent years in California we've had very warm conditions and very late starts to the wet season. What that has meant is there are record dry vegetation conditions when these autumn winds blow.

And so even without any change in the frequency or strength of those winds, we're seeing elevated risk coming from the added heat in the climate system that's causing there to be higher temperatures in the summer and autumn, and greater aridity of the vegetation. When that dry vegetation is combined with very strong winds, that creates a very high wildfire risk and very extreme wildfire behavior like we've been seeing in California in recent years.

MANN

You take extreme heat, you take extreme drought, you put them together, you get wildfire. This isn't rocket science. What we're seeing now here in the United States and what we're seeing simultaneously in Australia right now is symptomatic of these changes that we predicted decades ago. Our predictions are coming true, unfortunately.

MARKEYA

Thank you all. Well, those are all my questions. What is the best way for people to reach you if they have further questions?

MANN

I can be found on Twitter, on social media, and MichaelEMann at Twitter. My website is www.michaelmann.net and I'm MichaelMannScientist on Instagram, so come find me. I love talking about this stuff on social media as well.