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In the last section, we talked about the increase in greenhouse gases due to human activities over the last few hundred years. Combined with our knowledge of how these gases work from earlier in the class, our working hypothesis is that they lead to an increase in the temperature of the planet by increasing absorption of upwelling radiation and emission of downwelling longwave radiation to the surface. But is that really what we see when we look at observations?

Air temperature is the climate variable most readily observed with the most expansive historical record. Just think of how many things you know measure air temperature. Your cell phone, your car, even your local bank!

https://www.fosters.com/story/news/2015/01/09/bone-chilling-cold-but-nothing/34923658007/

Lucky for us, surface air temperature is relatively easy to measure and is a variable that is (almost) always reported anytime someone makes a weather observation. Instrumented surface air temperature measurements consisting of thermometer records from land-based stations, including islands, and ships provide us with more than a century of reasonably good global estimates of surface air temperature change. There is a particularly high-density network in places where people have lived, but even rural areas generally have samples from farmers and ranchers (and passionate amateur meteorologists!). Some regions, like the Arctic and Antarctic, and large parts of South America, Africa, and Eurasia – either due to a lack of people or means for temperature measurement – were not very well sampled in earlier decades, but records in these regions become available as people moved into them in the mid and late 20th century. Today, we now cover the planet with surface air temperature measurements using an expansive observer network.

So, what do our observations of air temperatures tell us when it comes to looking at how they change over time? Remember way back to our early lectures — air temperature variations are typically measured in terms of anomalies relative to some base period. Using anomalies helps us see changes over time more clearly, especially in climate data. Since absolute temperatures can vary widely between locations, comparing against a standard base period (like 1951-1980) gives us a clearer view of how much each region has warmed or cooled over time.

The animation below is taken from the NASA Goddard Institute for Space Studies in New York City (which happens to sit just above “Tom's Diner” of Seinfeld fame), one of several scientific institutions that monitor global air temperature changes. It portrays how temperatures around the globe have changed in various regions since the late 19th century. The temperature data have been averaged into 5-year blocks, and reflect variations relative to a 1951-1980 base period. Red regions are warmer than the 1951-1980 average, whereas blue regions are colder than the 1951-1980 average, by the amounts shown. You may note a number that appears in the upper-right corner of the plot. That number indicates the average temperature anomaly over the entire globe at any given time, again, relative to the 1951-1980 average.

Video: Surface Temperature Patterns (1:19)

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We can average surface air temperatures over the entire globe for any given year and get a single number, the global average surface air temperature. If we plot this temperature over time, we get a time series that shows how Earth's temperature has changed. This metric represents an average of temperatures from thousands of locations across the planet that helps us quickly understand overall warming or cooling trends. In the plot below, the temperature during a base period has been added to the anomalies, so each value represents the actual global average surface air temperature of the Earth for a year, rather than just the anomalies. However, whether we plot the actual temperatures or just their anomalies, the overall shape of the graph remains the same.

Trend in Global Average Surface Air Temperature. The grey dots and line are the actual observed year-to-year global average surface air temperatures while the black line is a "smoothed" version that represents averages over many years and removes year-to-year variability so we can isolate longer-term trends. "Lowess smoothing" is the method by which the black line was created from all the grey dots. More details here if you aren't scared off by statistics!

Credit: Global Land-Ocean Temperature Index, NASA's Goddard Institute for Space Studies (GISS) (Public Domain)

Since we began recording reliable, widespread temperature data in the late 1800s through land and ocean measurements, we’ve observed that Earth has warmed by about 1°C (or a little over 1.5°F). It’s important to emphasize—scientifically, this warming is undeniable. No matter how we slice the data, from different measurements or averaging methods, the long-term picture remains the same: Earth is warmer as we stand today than it was during the year of the first baseball World Series.

While the black line on the graph (showing the long-term trend) consistently rises from left to right, the grey line (representing the actual yearly observations) wiggles up and down quite a bit. You might notice that from the 1940s to the 1970s, temperatures appear relatively flat, even slightly cooler than their surrounding times. And you're not imagining things! In the mid-1970s, a few scientists thought we could be entering a long-term cooling phase. They had some reasons to believe this, such as changes in Earth's orbit that might eventually lead to an ice age, and the cooling effects of atmospheric aerosols. But at the time, it wasn’t clear how these cooling factors would compare to the warming influence of greenhouse gases.

Mythbusting

Now, you might hear some people asking: if scientists once thought we were heading into an ice age, why should we believe what they say about global warming now? Well, it's important to know that back in the '70s, the idea of an impending Ice Age wasn't a widespread belief among scientists. Think of it as more of a possibility that was being discussed, not a settled consensus. Most scientists believed that the effect of increasing greenhouse gases would likely be more significant, leading to warming, not cooling. Scientific consensus represents the prevailing view within the scientific community, based on the best available evidence at that time. Even though some scientists discussed cooling, the majority concluded that greenhouse gases would likely drive warming over the coming decade. Check out this graph from an article published in the Bulletin of the American Meteorological Society (BAMS) in 2008. The article's authors went back and counted how many scientific papers they could find that discussed global cooling, warming, or neutral projections between 1965 and 1980. They found there wasn’t a strong consensus... in fact, even during this period when some scientists thought we might be heading into a new ice age (the blue bars/line), the majority still predicted warming (the red bars/line) over the coming decades!

The number of papers classified as predicting, implying, or providing supporting evidence for future global cooling, warming, and neutral categories as defined in the text and listed in Table I. During the period from 1965 through 1979, our literature survey found 7 cooling, 20 neutral, and 44 warming papers.

Credit: © Copyright 2008 American Meteorological Society (AMS). For permission to reuse any portion of this Work, please contact permissions@ametsoc.org. 

And as it turned out, the brief cooling trend that had affected the Northern Hemisphere stopped in the '70s, and since then, global warming has been the dominant climate influence. See the image below with the global average surface air temperature anomalies post-1970 colored in red. With the clear and obvious warming, the small group of scientists pondering an impending ice age agreed that the long-term trend was indeed not cooling.

Northern Hemisphere Continental Temperature Trends.

Credit: Mann & Kump, Dire Predictions: Understanding Climate Change, 2nd Edition © 2015 Pearson Education, Inc.

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Now, even though we are starting to build a compelling narrative, we cannot deduce the cause of the observed warming solely from the fact that the globe is warming. Perhaps the sun is getting brighter. Maybe an army of underground gnomes have discovered space heaters! In our quest to understand why the warming is occurring, we can look for possible clues. Just like a detective, climate scientists refer to these clues as 'fingerprints.' In climate science, ‘fingerprints’ are distinct patterns or markers that reveal the causes of observed changes. These patterns help us differentiate between natural climate influences (like volcanic eruptions) and human activities (such as greenhouse gas emissions). It turns out that natural sources of warming give rise to different patterns of temperature change than human sources, such as increasing greenhouse gases. This is particularly true when we look at the vertical pattern of warming in the atmosphere.

Unlike surface air temperatures, estimates of upper air temperatures (from weather balloons and satellites) have only been available for the latter part of the past century. Still, they reveal something remarkable! The lower part of the atmosphere—known as the troposphere, where we live—has been warming along with the surface. But when we look higher up in the atmosphere, particularly in the stratosphere, air temperatures have been dropping! Take a look at the figure below. The four timeseries on the left show global temperature changes from the stratosphere (top panel) down to the troposphere (middle two panels), and finally to the surface (bottom panel). The bottom three panels show consistent warming, but in the upper stratosphere, we see a clear cooling trend. In the "Lower Stratosphere Temperature" panel, you’ll notice the temperature drops from left to right, meaning it’s been getting cooler over time. So, what's going on?

Recent Global Average Air Temperature Trends at Various Levels in the Atmosphere. These graphs show observed temperature trends at various altitudes in the atmosphere, from the lower stratosphere to mid to upper troposphere to lower troposphere to the surface. The graphic on the right shows the pattern of 20th century atmospheric air temperature changes predicted by climate models. Note that the greatest warming is observed in the tropicals lower atmosphere.

Credit: Mann & Kump, Dire Predictions: Understanding Climate Change, 2nd Edition © 2015 Pearson Education Inc.

Tropospheric Warming

Let’s focus on the warming trend in the lower atmosphere, known as the troposphere, which provides "part one" of our evidence for human-driven climate change. Greenhouse gases like CO₂ act like an insulating blanket around Earth, trapping warmth in the lower atmosphere, or troposphere. Here’s how: as the sun heats Earth, the surface emits some of that energy as infrared radiation. Greenhouse gases absorb this infrared energy and re-radiate it in all directions—including back toward the surface—creating a cycle of heat retention. With rising greenhouse gas levels, this trapping effect intensifies, warming the troposphere more and more. Because these gases have their highest concentrations within the lower atmosphere, the warming signal from additional greenhouse gases is expected to be concentrated near the surface, where the heat is initially captured and held.

Stratospheric Cooling

Now, for "part two," let’s take a look higher up—above where planes cruise—to the stratosphere. Unlike the warming we see in the troposphere, the stratosphere is actually cooling, and there are a couple of key reasons for this. First, because greenhouse gases trap heat in the troposphere, less heat makes it up to the stratosphere. Remember, these gases don’t create heat; they simply hold onto and re-radiate it... so the upper layers receive less energy. Second, remember when we discussed "other GHGs" and said that gases like halogenated and fluorinated compounds can break down ozone in the stratosphere? Ozone is crucial here because it A) absorbs solar radiation and B) likes to naturally hang out in the stratosphere for the most part (ozone does occur near the surface, but it's almost all due to pollution). With less ozone available, less sunlight gets absorbed in those layers, and less heat is generated in the stratosphere. Without this protective ozone layer to trap energy, more radiation simply passes through, continuing toward the surface instead of warming the upper atmosphere.

The simultaneous warming of the troposphere and cooling of the stratosphere is like a clear "tell" in climate’s version of poker—a move that unmistakably points to human-driven climate change. Natural factors, like an increase in solar energy, would either warm or cool all layers of the atmosphere at once. But when we examine temperature trends over the past 75 years or so in reanalysis data (our best historical reconstruction of the atmosphere), a distinct pattern emerges (see figure below): red, or warming, near the ground (below the dashed line marking the troposphere) and blue, or cooling, higher up in the stratosphere. Now, this isn’t a flawless fingerprint—there are still some warm areas higher up, mostly due to ozone changes in the Southern Hemisphere’s stratosphere. However, this overall pattern—warming below and cooling above—looks exactly like what we’d expect if greenhouse gases were the primary influence. This unique distribution aligns perfectly with our understanding of how greenhouse gases shift Earth’s energy balance, reinforcing the link between human activity and observed atmospheric changes.

Linear trend in zonally-averaged atmospheric air temperatures from 1948 to 2024. The x-axis represents latitude (with the North Pole on the right), and the y-axis represents atmospheric pressure (1000 hPa at the surface, 50 hPa is well into the stratosphere). Red indicates warming trends, while blue shows cooling trends. The dashed black line marks the approximate location of the tropopause, the boundary between the troposphere and stratosphere.

 

Credit: Colin Zarzycki, using data from the NCEP reanalysis

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The way most people feel the effects of climate change isn’t through gradual shifts in average temperatures but rather in the increased frequency and intensity of extreme, often severe weather events. These are broadly termed “extreme weather.” Extreme weather can mean (but this isn't an exhaustive list by any means!) things like hurricanes, intense heatwaves, prolonged droughts, heavy rainfall and flooding, severe wildfires, and extreme winter storms.

The link between climate change and extreme weather grabs a lot of attention—and for good reason. As we've learned over the past few lessons, climate change is complex and multi-layered, with influences ranging from human activities at local scales to natural and human-induced changes on a global level. Most news stories focusing on climate change and extreme weather are actually zooming in on one key aspect: human-driven global warming. The central question is often, “How does a warming Earth impact the weather we experience on a day-to-day basis?”

However, this question often gets rephrased in a misleading way—“Did climate change *cause* this heatwave, flood, drought, or storm?” Framed this way, the answer is “no.” Climate change doesn’t directly cause specific weather events. That is, we can't unequivocally say Hurricane Katrina only happened because of climate change, and otherwise, it would be a perfectly pleasant sunny day! Blistering heatwaves, severe thunderstorms, devastating floods, and powerful hurricanes all existed long before human influence on the climate. So, while climate change doesn’t directly trigger a specific heatwave or flood, scientists' critical questions are whether and how climate change makes these extreme events more intense, frequent, impactful, or likely to happen in certain regions.

This is a complex area of study, and the science connecting climate change with extreme weather is rapidly evolving -- this lesson may not even be the same year-to-year! What is clear, though, is that the impacts of climate change on extreme weather differ from place to place. Although the globe is warming, some areas are heating up faster than others; similarly, while sea levels are rising, local factors cause them to rise unevenly across different coastlines. In short, a mix of local, regional, and global climate influences—both human-induced and natural—play into changing patterns of extreme weather. Yet, the human-driven warming of the oceans and atmosphere has certainly altered some aspects of atmospheric behavior.

Let's start with a couple of the more straightforward connections between global warming and extreme weather events. For starters, as the world has warmed, average air temperatures in many areas have increased. Not surprisingly, so have outbreaks of hot weather. On a similar note, episodes of extremely cold weather have declined, which seems intuitive.

As the climate has warmed, average temperatures have increased, as the entire temperature distribution has shifted warmer. This leads to fewer occurrences of extremely cold weather, but more occurrences of extremely hot weather.

Credit: Intergovernmental Panel on Climate Change, 2007

Take a look at the graph above. It shows the probability of experiencing “cold,” “near-average,” and “hot” weather. Notice that the “previous climate” curve has a “bell” shape—this is because, in a stable climate, it’s most common for temperatures to be seasonable or close to average. However, as anyone who’s been around for a few years knows, we also get stretches of both very cold (left side of the curve) and very warm (right side of the curve) weather.

Now, look at the “new climate” curve, which shows what happens when the entire temperature distribution shifts warmer. Every temperature is nudged a bit to the right, making warmer temperatures more likely. Remember our discussion of statistical distributions? What we’re seeing here is an example of how shifting a distribution affects the likelihood of different outcomes. This shift means that, while seasonable weather is still most common, “average” itself has become a little warmer. For example, if May’s average high temperature used to be around 65°F, it might now be closer to 68°F.

So, what’s the result of this shift? With the whole distribution skewed a bit warmer, extreme cold events (on the left) become less common, and extreme heat events (on the right) become more common. It makes sense—by shifting the baseline, we’re tilting the odds toward hotter days.

This doesn’t mean cold spells will vanish altogether as the world warms. Cold snaps can still happen! Take February 2015, for instance, when the eastern United States had one of its coldest months on record (going back to 1895). So, yes, even in a warming climate, frigid weather can occasionally dominate. But when we look over a long period, we start to see a trend: fewer cold spells and more frequent heatwaves.

Do we actually see more extreme heat days compared to extreme cold days? Let’s explore that by looking at decade-by-decade data on the frequency of record-breaking warm and cold days, totaling up all U.S. locations and days of the year. This is what the trend looks like through 2015.

Figure 3.31: Change Over Time in the Relative Incidence of Warm and Cold Daily Extremes.

Credit: Record Highs vs. Record Lows Climate Central (Used with permission)

This graph illustrates the ratio of daily record high temperatures to record low temperatures over the decades. Each bar's color and height provide insights into temperature trends:

• Red Bars: More record highs than lows.
• Green Bars: More record lows than highs.
• Bar Height: Indicates the extent of the difference. Taller bars signify a greater disparity between record highs and lows.

In a stable climate, we’d expect the number of record highs and record lows to be pretty even. And that was indeed the case through the mid-20th century: if you look at the red bars in the graph, they’re relatively short, indicating a near 1:1 ratio. In other words, it was about a coin flip whether record highs or lows were more common.

But then came the 1960s and 1970s, when mean temperatures in the Northern Hemisphere actually cooled slightly. This cooling led to more record lows than highs, as you can see with the green bars that pop up. Since then, however, there’s been a sharp shift. As the global temperature trend turned upward, particularly in the Northern Hemisphere and North America, record highs began to outnumber record lows significantly. In the past decade, the ratio of record highs to lows has soared to over 2:1, meaning extreme heat days now occur twice as often as extreme cold days. This trend aligns with the shift in the probability of heat extremes we discussed earlier.

A helpful analogy here is to think about rolling a six-sided die. Yes, yes, we’ve played with this example earlier in the semester, but let’s roll with it again (pun intended)! Let’s set up another little experiment with die rolling to see how probabilities change.

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Another outcome of a warming atmosphere and ocean is increased moisture. Earlier, we discussed how rising temperatures lead to higher evaporation rates. Eventually, condensation rates catch up, reaching a new balance, but in this warmer state, the number of water vapor molecules in the air is greater. In the atmosphere, this has a major impact: if warmer air holds more water vapor, then when it rises and cools to the point where net condensation occurs, there’s more water available for cloud formation and precipitation.

To gauge the moisture available for precipitation, climate scientists use a metric called "precipitable water." This represents the amount of rain that would fall if all the water vapor in a column of air—from the Earth's surface up to the top of the troposphere—condensed and fell as rain. The image below shows the simulated percentage change in precipitable water between the 1984-2013 average and the 1871-1900 average. The widespread blue shading indicates that in most regions, precipitable water has increased by several percent, and in some areas, by as much as 15 percent as the world has warmed.

The difference between the 1984-2013 average of atmospheric precipitable water and the 1871-1900 average shows that most areas of the globe have had an increase in precipitable water ranging from a few percent to 15 percent or more.

Credit: NOAA

This trend has critical implications for precipitation intensity and frequency, as more moisture in the atmosphere can lead to more intense downpours, amplifying the risk of floods and extreme rainfall events in many areas. Therefore, it's not surprising to see an increase in heavy rain events. For example, the percentage of the contiguous United States receiving an unusually large portion of total annual rainfall from extreme one-day rainfall events has increased (here's the corresponding graph from NOAA; the orange curve represents a running nine-year average). But, the increase in heavy rain events hasn't occurred equally everywhere.

The frequency and intensity of heavy precipitation events have increased across much of the United States, particularly the eastern part of the continental US, with implications for flood risk and infrastructure planning. Maps show observed changes in three measures of extreme precipitation: (a) total precipitation falling on the heaviest 1% of days, (b) daily maximum precipitation in a 5-year period, and (c) the annual heaviest daily precipitation amount over 1958–2021. Numbers in black circles depict percent changes at the regional level. Data were not available for the US-Affiliated Pacific Islands and the US Virgin Islands.

Figure credits: (a) adapted from Easterling et al. 2017;282 (b, c) NOAA NCEI and CISESS NC. Source: https://nca2023.globalchange.gov/chapter/2/

The figure above, from the U.S. National Climate Assessment, highlights trends in “heavy precipitation”—defined here as the top one percent of all rainfall events in each region—from 1958 through 2021. In the leftmost panel (the one labeled "a"), we can see that the Northeast has experienced the largest increase in rainfall from these intense events, while the Pacific Northwest has seen a smaller increase, and Hawaii actually shows a slight decrease in rainfall from its heaviest events during this period.

The other two panels display different measures of extreme precipitation, such as the maximum daily rainfall over one- and five-year windows. The overall story remains consistent: significant increases in the eastern United States, with smaller yet still notable, increases in the West. These trends aren't limited to the United States; as we’ll see shortly, similar patterns are evident globally, with variations across regions.

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Remember when we discussed how the general circulation of the atmosphere—the global pattern of winds and weather—is shaped by the uneven heating of Earth? Because the equator receives more solar energy than the poles, this imbalance drives a vast system of atmospheric circulations that distribute heat and moisture around the planet. So, when we talk about climate change, we’re not only talking about shifts in temperature; we’re also talking about changes in this energy imbalance that can impact circulation patterns. It’s logical, then, to expect climate change to influence large-scale precipitation trends as well.

We just explored how a warmer atmosphere holds more moisture, which translates into heavier downpours when that moisture eventually condenses. In the U.S., the data confirm this trend toward more intense rainfall events. However, precipitation patterns are far from simple. It’s one thing to say heavier downpours are on the rise, but predicting how total precipitation will change in any given place—how much rain or snow falls over an entire season or year—is much more complex. Precipitation changes are “noisier,” meaning they vary a lot by location and can be harder to interpret consistently over time.

Why is precipitation so variable? The reasons range from local topography to larger-scale climate patterns. For instance, mountain ranges can trap moisture on their windward sides, creating wetter conditions there, while casting a “rain shadow” of drier conditions on their leeward sides. Oceans also play a big role: they support massive air and water currents, like the monsoons, which bring seasonal rain to regions like South Asia and East Africa. Ocean-atmosphere interactions are critical, and when they shift even slightly, they can change rainfall patterns for entire regions.

Unlike temperature, which has shown a more uniform warming trend globally, precipitation trends are a mixed bag—some regions are seeing more rain, while others face more frequent droughts. These changes don’t follow the same clear patterns as temperature, which is why scientists continue to investigate how climate change impacts precipitation in different parts of the world. This research is crucial for understanding the full picture of change impacts.

Now, let’s zoom out and take a more global look at total precipitation patterns. The figure below maps how different areas have seen precipitation either increase (in green) or decrease (in brown) over the 20th century. The small red dots mark different locations, each with its own time series showing how precipitation has trended over the years.

Trends in Annual Precipitation, 1901-2005 [Enlarge].

Credit: IPCC Fourth Assessment Report, Chapter 3, Figure 3.14

Pretty messy, right? Some areas show clear trends, but they’re all over the map. Take southern South America, for example—the time series in the bottom left corner shows a noticeable increase in rainfall across the century, indicating that this region is becoming wetter. On the other hand, regions like southern Asia and India (fourth row, third column) show a drying trend, especially over the past 30 years. These diverse precipitation signals across the world are essential to understand because climate change isn’t affecting every region equally. As global conditions shift, the “winners and losers” in terms of rainfall and drought may shift too. Places that historically supported lush forests and agriculture may face more frequent droughts, while regions previously considered too dry for intensive farming might start seeing conditions that favor crops. By the end of this course, we’ll dive into what these shifts could mean for geopolitics and resource distribution.

While these precipitation patterns are complex and heavily influenced by local factors, we do start to see some overarching trends. Remember the Clausius-Clapeyron relationship? It’s that principle telling us that a warmer atmosphere holds more water vapor before it condenses into cloud droplets and, eventually, rain. This relationship suggests that with a warming climate, regions near the equator—where the Hadley cell drives constant upward motion—might get wetter over time. Why? A warmer, moisture-laden lower atmosphere in equatorial regions means more available water vapor for rain.

So, does this theory hold up in real-world data? When we look at precipitation trends by latitude, we start to see supporting evidence. By averaging observed precipitation changes across latitude bands, a pattern emerges: certain regions, especially near the equator, are indeed becoming wetter over time.

Changes over Time in Precipitation for Various Latitude Bands

Credit: IPCC Fourth Assessment Report, Chapter 3, Figure 3.15

Take a look at the broad green band around 15°N latitude on the graph. This region aligns closely with the Intertropical Convergence Zone (ITCZ), where equatorial rain belts form due to warm, rising air. Seeing a green band here suggests that rainfall in the ITCZ has increased, which matches what we’d expect as the atmosphere warms and holds more moisture. Meanwhile, higher latitudes show a trend toward drying, represented by yellows and browns near the top and bottom of the graph. These drying patterns are consistent with some climate model predictions (we’ll dig into those next), but precipitation trends are notoriously variable. From year to year and even decade to decade, regional precipitation can swing widely, making it hard to confirm clear, theory-matching patterns like we saw with temperature, sea ice, or sea level.

An essential factor in this story is drought—periods of unusually low rainfall. But drought isn’t just about less rain; it also depends on temperature. Imagine this: when rainfall drops, the ground heats up faster, which speeds up evaporation and further dries out soil. This reduction in soil moisture limits the water available for plants, creating harsher conditions for growth. This process can even reinforce itself, with drier soil warming up faster, leading to even more moisture loss—a process often described as “drought begets drought.”

Drought patterns, like precipitation, are complex and vary by region. Yet, we’re seeing droughts become more common in some areas, even in regions where rainfall hasn’t decreased significantly. Warmer temperatures alone can drive moisture out of the soil, which is especially clear when we look at tools like the Palmer Drought Severity Index (PDSI). The PDSI is a numeric index for soil moisture that factors in both temperature and precipitation. The more negative the PDSI number, the stronger the drought.

Check out the time series of the PDSI over the U.S. from 1890 to 2020 below. Do any patterns stand out? Notice the deep drop in the 1930s—this was the infamous Dust Bowl, when a severe drought brought destructive dust storms, devastating agriculture across the American and Canadian prairies. There’s another notable drought in the 1950s, but since then, we haven’t seen anything as extreme. What’s important to note here is that droughts haven’t disappeared despite increases in intense rainfall events. Just because we expect heavier downpours as the atmosphere warms doesn’t mean droughts are off the table in U.S. climate trends. While researchers are still examining historical data, I’d argue it’s too early to definitively say climate change is tilting the scales toward more or fewer droughts in the long run—at least not yet.

Palmer Drought Severity Index from 1890 to 2020, showing fluctuating drought severity with a baseline at zero.

Credit: Climate Change Indicators: Drought, Environmental Protection Agency (EPA) (Public Domain)

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While trends in extreme heat, cold episodes, and heavy rainfall have shown some global variation, scientists are confident that global warming significantly influences these patterns. However, determining how global warming impacts smaller-scale or short-lived storms—like hurricanes (tropical cyclones), mid-latitude low-pressure systems (extratropical cyclones), and severe thunderstorms (convective storms)—is more complex.

The figure below provides a sense of the certainty levels regarding links between global warming and different types of extreme weather events. Those toward the upper right of the graph represent trends with higher confidence in their connection to global warming, while those in the bottom left indicate areas with less certainty. Notably, the graph includes "extreme rainfall" but not "flooding" as a separate category. Trends in flooding are highly localized, influenced by factors such as urbanization, which affects water absorption and drainage into nearby streams and rivers. Poor urban planning, for instance, can lead to increased flooding regardless of trends in heavy rainfall. This means that while global warming may influence extreme rainfall, local land-use changes often play a major role in determining flooding outcomes, making it challenging to link flooding trends directly to climate-driven changes in rainfall.

Scientists are most confident in linking changes in episodes of extreme heat and extreme cold to global warming, followed by droughts and extreme rainfall. Confidence in attributing changes to smaller, shorter-lived atmospheric phenomena like tropical cyclones (such as hurricanes), extratropical cyclones (mid-latitude low-pressure systems), and severe convective storms (thunderstorms) is lower.

Credit: National Academy of Sciences

Interest in the links between global warming and extreme storms like hurricanes, severe thunderstorms, or tornadoes is high. However, there’s less certainty in understanding these connections. Let’s focus on tornadoes and tropical cyclones (hurricanes), since they’re often in the spotlight. Part of the challenge with linking these storms to global warming is the relatively short period of quality observations we have of them.

For instance, while the number of strong tornadoes in the U.S. hasn’t shifted much since 1950, the frequency of weak tornadoes has risen significantly (see the graph below). Is this increase due to global warming? Probably not. Instead, the rise is largely due to improved storm detection with the introduction of NEXRAD Doppler radar in the early 1990s, which allows for more accurate tracking of storms that might produce tornadoes. Prior to advanced radar, only tornadoes easily observed and reported from the ground were included in the records—meaning in parts of the central U.S. where much of the landscape is dominated by acres and acres of crops instead of humans, weaker tornadoes likely went undetected and/or reported!

The number of tornados observed per year over the United States, broken down by Fujita scale intensity.

Credit: Ian Livingston / ustornadoes.com

A similar story applies to tropical cyclones. From 1980 to 2017, hurricanes dominated the list of costliest U.S. weather disasters (credit: NCEI), underscoring their substantial societal impact. But is this impact due to human-induced warming making hurricanes more frequent or intense? It’s complicated. Reliable observations only go back to the 1970s when global satellite coverage began; prior to that, storms that didn’t make landfall or encounter ships often went unrecorded. Adjusting for this, the data suggest the overall number of tropical cyclones worldwide has changed very little with warming, although a higher percentage of hurricanes are reaching extreme strength (sustained winds over 110 mph). Still, the rise in hurricane damage appears to be driven more by increased coastal development—more people and infrastructure to be affected—than by warming itself.

Yet, some trends consistent with global warming are evident. For instance, rising sea levels amplify coastal flooding during tropical cyclones. Also, the areas where tropical cyclones develop are expanding, and these storms are reaching peak intensities farther from the equator than a few decades ago.

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We’ve covered many extreme weather and climate concepts from a U.S. perspective, but how do these trends play out globally? The figure below (also, big version linked here) from the IPCC Sixth Assessment Report divides the world into regions (no, this isn’t a game of Settlers of Catan!). Each “tile” represents a region, such as ENA for Eastern North America. A team of scientists assessed changes in heat extremes, heavy precipitation, and drought since the 1950s, topics we’ve explored in this lesson. Tile colors indicate observed trends, while the number of dots represents the confidence scientists have that these trends are driven by human activity.

The top panel displays trends in hot extremes and confidence in human contributions. Red tiles show increases in hot extremes, blue shows decreases, dashed lines indicate unclear trends, and gray signifies limited data. Most regions show red tiles, supporting our earlier discussions on rising ratios of record highs to lows. High confidence (three dots) in many regions links these increases to greenhouse gas emissions and other human impacts.

The middle panel shows trends in heavy precipitation, with green for increases and brown for decreases. Most tiles are green, with trends pointing to more intense precipitation, as we discussed in North America. However, gray tiles in South America and Africa reveal gaps in data, underscoring why scientists advocate for better monitoring networks.

The bottom panel illustrates trends in agricultural and ecological drought (prolonged low precipitation). While heavy precipitation events have increased, drought trends are mixed, but most tiles are brown, indicating rising drought levels. How can heavy precipitation and drought both be on the rise? Timing is the key. In warmer climates, precipitation events cluster together, leading to long dry periods that stress water resources—a phenomenon known as the “boom-bust” cycle, where we see increasing shifts between very wet and very dry conditions.

Global assessment of climate change impacts across regions (1950s-present) showing observed changes in (a) heat extremes, (b) heavy precipitation, and (c) agricultural/ecological drought. The hexagonal grid represents different geographical regions using IPCC AR6 WGI reference regions. Color coding indicates the type of change (increase/decrease), while dot patterns show the confidence level in human contribution to these changes. Red hexagons in (a) show widespread increases in hot extremes, green hexagons in (b) indicate increases in heavy precipitation in some regions, and yellow hexagons in (c) show drought increases in selected areas. Gray hexagons indicate limited data availability, while white hexagons represent low agreement in observed changes

Source: https://www.ipcc.ch/report/ar6/wg1/figures/summary-for-policymakers/figure-spm-3/

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Summary

• Surface temperature is easy to measure, and global observations confirm a steady warming trend over the past century.
• The global average temperature provides a single, useful metric for assessing Earth’s warming.
• Distinct "fingerprints" of human-driven climate change, such as surface warming and stratospheric cooling, support the case for anthropogenic impacts.
• Ocean warming and acidification are directly impacting marine life, with potentially far-reaching effects on global food security.
• Melting sea ice and land ice indicate regional warming differences, with distinct impacts depending on the ice source.
• Sea levels are rising globally due to ice melting and thermal expansion, though regional variations exist due to local factors.
• Global warming shifts temperature extremes, increasing the likelihood of record highs and diminishing the frequency of record lows.
• A warmer atmosphere holds more moisture, leading to intensified heavy rainfall events, with regional variations in frequency and impact.
• Shifts in global precipitation patterns (along with accompanying floods and droughts) vary widely, influenced by factors like geography, ocean currents, and climate change.
• Confidence varies in linking global warming to different extreme weather events, with particular challenges for short-lived phenomena like tornadoes and hurricanes.
• Global trends in extreme heat, heavy precipitation, and drought reveal regional differences, with data visualization strategies (like the hexagonal plots we explored) enhancing our understanding of human influence.