This lecture discusses impacts to extreme events. All extreme events have many different causes. It's impossible to determine if a natural event was directly caused by climate change.
However, researchers have started trying to understand how much of an event might have been impacted by climate change. They can try to figure out how much more severe the event was as a result of climate change. This is like quantifying how much weight loss might have happened from diet and how much from exercise. This type of analysis is not a forecasting technique.
It can only analyze events that have already happened. However, if we can figure out if events might be bigger than before, then we can help communities prepare for the events. The analyses are primarily based on statistics.
The research can determine if the timing of the events have changed, if the events are shorter or longer, or if the events are larger or smaller in magnitude. Percentiles are a statistical technique used to identify a frequency distribution. If all events are evaluated and then organized into groups that show every 10% change, then one single event or one single year can be evaluated for what group it's in. For example, it's a way to see if an event is in the top 10% or the bottom 10% of all events. Standard deviations are also called sigmas.
They show how far an event might be away from the statistical mean of the observations. A statistical mean is just an average. If the average amount of weight lost in a group of friends is 10 pounds, then the 20 pounds lost by one friend would be several sigmas away from the group's average. The more sigmas, the more unusual the event. A recurrence interval is a statistical relationship that describes how often an event might occur.
A recurrence interval is also called a return period. A 100-year flood describes a historical amount of water in a channel that statistically happens only once in a 100-year period. It's not a forecasting technique.
It's just a way to understand how often a certain magnitude tends to For a 100-year return interval, that means there's a 1 in 100 chance or a 1% chance that the event could occur this year. A probability is simply a calculation of the event divided by time. That means there's a 99% chance that it won't happen. The longer the time span, the more likely the event will occur.
After 500 years, it's more than likely that the the event will happen. The first time the scientific community was able to make some connections between extreme weather events and climate change came in the 2016 National Academy of Science report. Since warmer atmospheres can evaporate more water, it makes sense to see some trends on floods and drought.
Heat waves also seem to be increasing in magnitude and duration. However, there are no connections between climate change and tornadoes, hurricanes, and wildfires. Wildfires are more often impacted by poor land management. The numbers of hurricanes globally have been decreasing.
What may be happening is while there might be fewer hurricanes, the ones that do occur may be more extreme. Think about Harvey or Maria from 2017 or Florence from 2018. The United States has 10 times more tornadoes than anywhere else in the world. Tornadoes have been trending down in the last few years. There really isn't an explanation for this trend at this time.