The Basics of Climate Change

 

11

If the world is warming, why are some winters and summers still very cold?

Global warming is a long-term trend, but that does not mean that every year will be warmer than the previous one. Day-to-day and year-to-year changes in weather patterns will continue to produce some unusually cold days and nights and winters and summers, even as the climate warms.

Climate change means not only changes in globally averaged surface temperature, but also changes in atmospheric circulation, in the size and patterns of natural climate variations, and in local weather. La Niña events shift weather patterns so that some regions are made wetter, and wet summers are generally cooler. Stronger winds from polar regions can contribute to an occasional colder winter. In a similar way, the persistence of one phase of an atmospheric circulation pattern known as the North Atlantic Oscillation has contributed to several recent cold winters in Europe, eastern North America, and northern Asia.

Atmospheric and ocean circulation patterns will evolve as Earth warms and will influence storm tracks and many other aspects of the weather. Global warming tilts the odds in favour of more warm days and seasons and fewer cold days and seasons. For example, across the continental United States in the 1960s there were more daily record low temperatures than record highs, but in the 2000s there were more than twice as many record highs as record lows. Another important example of tilting the odds is that over recent decades heatwaves have increased in frequency in large parts of Europe, Asia, South America, and Australia. Marine heat waves are also increasing.

 

12

Why is Arctic sea ice decreasing while Antarctic sea ice has changed little?

Sea ice extent is affected by winds and ocean currents as well as temperature. Sea ice in the partly-enclosed Arctic Ocean seems to be responding directly to warming, while changes in winds and in the ocean seem to be dominating the patterns of climate and sea ice change in the ocean around Antarctica.

Some differences in seasonal sea ice extent between the Arctic and Antarctic are due to basic geography and its influence on atmospheric and oceanic circulation. The Arctic is an ocean basin surrounded largely by mountainous continental land masses, and Antarctica is a continent surrounded by ocean. In the Arctic, sea ice extent is limited by the surrounding land masses. In the Southern Ocean winter, sea ice can expand freely into the surrounding ocean, with its southern boundary set by the coastline of Antarctica. Because Antarctic sea ice forms at latitudes further from the South Pole (and closer to the equator), less ice survives the summer. Sea ice extent in both poles changes seasonally; however, longer-term variability in summer and winter ice extent is different in each hemisphere, due in part to these basic geographical differences.

Sea ice in the Arctic has decreased dramatically since the late 1970s, particularly in summer and autumn. Since the satellite record began in 1978, the yearly minimum Arctic sea ice extent (which occurs in September) has decreased by about 40% [Figure 5]. Ice cover expands again each Arctic winter, but the ice is thinner than it used to be. Estimates of past sea ice extent suggest that this decline may be unprecedented in at least the past 1,450 years. Because sea ice is highly reflective, warming is amplified as the ice decreases and more sunshine is absorbed by the darker underlying ocean surface.

Sea ice in the Antarctic showed a slight increase in overall extent from 1979 to 2014, although some areas, such as that to the west of the Antarctic Peninsula experienced a decrease. Short-term trends in the Southern Ocean, such as those observed, can readily occur from natural variability of the atmosphere, ocean and sea ice system. Changes in surface wind patterns around the continent contributed to the Antarctic pattern of sea ice change; ocean factors such as the addition of cool fresh water from melting ice shelves may also have played a role. However, after 2014, Antarctic ice extent began to decline, reaching a record low (within the 40 years of satellite data) in 2017, and remaining low in the following two years.

 

13

How does climate change affect the strength and frequency of floods, droughts, hurricanes, and tornadoes?

Earth’s lower atmosphere is becoming warmer and moister as a result of human-caused greenhouse gas emissions. This gives the potential for more energy for storms and certain extreme weather events. Consistent with theoretical expectations, the types of events most closely related to temperature, such as heatwaves and extremely hot days, are becoming more likely. Heavy rainfall and snowfall events (which increase the risk of flooding) are also generally becoming more frequent.

As Earth’s climate has warmed, more frequent and more intense weather events have both been observed around the world. Scientists typically identify these weather events as “extreme” if they are unlike 90% or 95% of similar weather events that happened before in the same region. Many factors contribute to any individual extreme weather event—including patterns of natural climate variability, such as El Niño and La Niña— making it challenging to attribute any particular extreme event to human-caused climate change. However, studies can show whether the warming climate made an event more severe or more likely to happen.

A warming climate can contribute to the intensity of heat waves by increasing the chances of very hot days and nights. Climate warming also increases evaporation on land, which can worsen drought and create conditions more prone to wildfire and a longer wildfire season. A warming atmosphere is also associated with heavier precipitation events (rain and snowstorms) through increases in the air’s capacity to hold moisture. El Niño events favour drought in many tropical and subtropical land areas, while La Niña events promote wetter conditions in many places. These short-term and regional variations are expected to become more extreme in a warming climate.

Earth’s warmer and moister atmosphere and warmer oceans make it likely that the strongest hurricanes will be more intense, produce more rainfall, affect new areas, and possibly be larger and longer-lived. This is supported by available observational evidence in the North Atlantic. In addition, sea level rise (see Question 14) increases the amount of seawater that is pushed on to shore during coastal storms, which, along with more rainfall produced by the storms, can result in more destructive storm surges and flooding. While global warming is likely making hurricanes more intense, the change in the number of hurricanes each year is quite uncertain. This remains a subject of ongoing research.

Some conditions favourable for strong thunderstorms that spawn tornadoes are expected to increase with warming, but uncertainty exists in other factors that affect tornado formation, such as changes in the vertical and horizontal variations of winds.