July 16, 2019 – One day, you see a forecast calling for sun, so you don’t bother to pack an umbrella. But just your luck: Clouds tumble in and suddenly you’re stuck in a rainstorm. Now you’re soaking and looking for someone to blame.

In this case, you’ve experienced the variability of weather. Weather is the mix of events that happen every day in our atmosphere. Meteorologists develop models to forecast atmospheric conditions based on past data. But weather isn’t the same thing as climate. Read on for critical points about the difference.

1. The difference between weather and climate

A meteorologist’s daily forecast focuses on weather. Meteorologists generally say that forecasts become less reliable if they go beyond a week or perhaps 10 days. Climatologists, on the other hand, define climate as the average weather over a 30-year period and over a much larger geographic area.

The climate of a place is characterized by the type, frequency, duration, and intensity of weather. (A cable TV host has joked that climate really is just a combination of thousands of “tiny weathers.”)

2. Think of climate as your personality and weather as your mood

Michael Mann, a Penn State professor of atmospheric science, compares weather to mood and climate to personality. Say you’re usually a friendly person. But one time, you run into an unexpected traffic jam and get road rage. Understandably, your mood fluctuates, so one extreme event won’t change your personality. But imagine you move to Los Angeles, get caught in traffic every day, and develop a habitat of yelling at passing cars. Now you might have a problem.

“Someone who becomes more and more irritable over time is experiencing a change in personality, just like a long-term drift toward warmer and more extreme weather reflects climate change,” Mann said. (Another common metaphor used by many climate science communicators goes like this: “Weather informs us on what clothes to wear that day. Climate informs us on what kinds of clothes we should have available in our wardrobes for use year-round.”)

Collectively, the increase in extreme weather can indicate a changing trend. “Just like a single data point in a sample of data contributes to the average of the data, every weather event contributes to the long-term average,” Mann said.

3. Humans are causing the climate to warm

Humans are changing the climate by releasing greenhouse gases into the atmosphere. Scientific analysis has shown that human activities have caused the Earth to warm about 1.0 degrees Celsius (1.8 degrees Fahrenheit) since large-scale industry began around 1750. In 2018, the Intergovernmental Panel on Climate Change reported that Earth will cross a key warming threshold of 1.5 degrees Celsius (2.7 degrees Fahrenheit) between 2030 and 2052, if “business as usual” greenhouse emissions continue.

Many evidence-based scientific findings suggest that climate change has already caused unusually hot summer days and nights to become more common since the 1970s. Hot summer days are especially dangerous for elderly people, children, outdoor workers, and the homeless. Nighttime temperatures are climbing at higher rates than daytime temperatures, which may also prevent those heat-sensitive populations from getting relief from the heat at night.

4. Climate change will change the nature of precipitation

It’s not just extreme temperatures on the rise. Climate change increases the intensity and frequency of drought. Heavy precipitation events are also growing more numerous as atmospheric temperatures increase.

Ahira Sánchez-Lugo, a climatologist with NOAA’s National Centers for Environmental Information, compares the atmosphere’s capacity to carry water to the size of a water cup. As the atmosphere warms, the atmosphere can hold more water vapor, as though the cup’s size is increasing. It takes longer to fill a bigger cup, which has some serious consequences.

“You’re going to have more days that are going to be dry, but when that cup is full, you’re going to be getting much more precipitation in an area that is not used to getting that type of precipitation in such a period of time,” Sánchez-Lugo said.

The effects of climate change may intensify extreme weather events like hurricanes. Widely accepted science says that warmer sea-surface temperatures may increase tropical wind speeds and that sea-level rise will worsen storm surge. Scientists haven’t observed an increase in the number of hurricanes, but research indicates that hurricanes are producing more rainfall.

5. You can blame climate change for contributing to some – but not all – extreme weather events

If you’ve experienced a particularly intense storm, you may have blamed climate change. But it’s not quite that simple. Climate change affects the odds of extreme weather, in effect “loading the dice” and increasing the likelihood of those severe events. Scientists engaged in a new field known as attribution science are trying to better quantify the relationship between the warming climate and individual severe weather events.

In this growing body of research, scientists compare simulated worlds using computer models: some in which climate change has occurred and others in which it has not. They then can compare the odds of an event’s happening in the real world with its happening in a world untouched by humans.

Scientists have found, for example, that climate change has increased the likelihood of heat waves in the eastern U.S. Other recent heat waves, such as the one in western Europe in the early summer months of 2019, have also been linked to climate change. Other specific extreme weather events are more difficult to attribute causally to climate change, though many scientists emphasize that changing conditions – higher atmospheric global average temperatures and increasing sea levels – in effect have “set the table” for more severe storms by increasing the background conditions or foundations from which storms originate and grow.

Brooke Bauman is an intern at YCC and a student at UNC-Chapel Hill studying environmental science, geography, and journalism. www.yaleclimateconnections.org