4.6 - Climate
As you remember, weather is what is going on outside your window right now. Climate is the long term observation of humidity, cloudiness, wind, precipitation etc. There is obviously a huge amount of information encompassed by the concept of climate. And as occurs in many fields (botany and zoology to name two), it is helpful to take the most important climate characteristics and classify climates in some way.
As far as we know, the Greeks were the first to attempt to classify climate 2200 years ago. They kept it simple: the temperate zone was the midlatitudes in which they lived, the torrid zone was the Tropics to the south, and the frigid zone was to the north. They knew the earth was round and they thought the Southern Hemisphere must contain the same zones.
In 1918, Wladimir Koppen developed what has served as the basis for most modern climate classifications which have followed. He was a climatologist and a botanist. The Koppen System uses a numerical basis of classification (temperature and precipitation averages) with many zone boundaries determined by climatic conditions associated with vegetation patterns.
The Koppen system recognizes five primary climate classifications : A (Tropical), B (Dry), C (midlatitude with mild winters), D (midlatitude with severe winters) and E ( Polar). The system adds second and third letters to a climate's description to provide a more precise definition of a climate. For example, in Sonoma County our climate classification is a Csa - midlatitude climate with a mild winter, a dry and hot summer. Mississippi is also a C climate but has no dry season and a hot summers (it is a Cfa)
We (coastal California) share our Csa or Mediterranean climate with the following areas: Central Chile, the Cape Town region of South Africa, the Mediterranean Basin, Southern and SW Australia. All of these locations are on the west side of continents , adjacent to cold ocean currents, located at about 35° to 40° north or south of the equator. The subtropical high dominates in these latitudes during the summer months, causing a seasonal drought.
B or dry climates cover about 30%of the land area of the world - more than any other climactic zone. The largest expanses of dry areas are in subtropical latitudes, especially in the central and western parts of continents where the subtropical high dominates year round. We also see dry regions in the center of large continents far from the ocean or on the lee sides of mountain ranges in their rain shadow.
Tropical humid climates (A climates) occupy almost all the land area of Earth within some 15-20° of the equator in both the Northern and Southern Hemispheres. In some places it extends poleward beyond 25°. These are true winterless climates with no cold period. They have moderately high temperatures throughout the year. Some of these climates have a distinct winter dry season (think monsoons), others have consistent precipitation throughout the year. They are primarily in the zone of the ITCZ, where air heated by high sun angles and consistent length of day near the equator rises, cools and produces abundant precipitation.
D climates (severe midlatitude climates) only occur in the northern hemisphere because the southern hemisphere has little land in the latitudes between 40° and 70° latitude. This latitude extends broadly across North America, much of Canada and Alaska. It includes Eurasia from eastern Europe through most of Russia all the way to the Pacific Ocean. These locations don't experience the moderating influence of the oceans. We see large temperature ranges here between winter and summer and there are four distinct seasons.
Lastly there are polar climates (E climates). The polar climates have low sun angles and variable lengths of day. No month averages above 50°F and locations with these climates, have the coldest summers and lowest annual temperatures.
Climate Change
Earth's climate has changed throughout it's history. Just in the last 650,000 years there have been seven cycles of glacial advance and retreat, with the end of the last ice age about 11,700 years ago marking the beginning of the modern climate era — and of human civilization. Most of these climate changes are attributed to very small variations in Earth’s orbit that change the amount of solar energy our planet receives.
The current warming trend is of particular significance because most it is extremely likely (greater than 95% probability) to be the result of human activity which has occurred since the mid-20th century. Warming is proceeding at a rate that is unprecedented over centuries to millennia.
Earth-orbiting satellites and other technological advances have enabled scientists to see the big picture, collecting many different types of information about our planet and its climate on a global scale. This body of data, collected over many years, reveals a changing climate.
The heat-trapping nature of carbon dioxide and other gases was demonstrated in the mid-19th century. We know that the "greenhouse" gases trap long wave terrestrial radiation. There is no question that increased levels of greenhouse gases result in the Earth warming.
Ice cores taken from Greenland, Antarctica, and tropical mountain glaciers show that Earth’s climate clearly responds to changes in greenhouse gas levels. Ancient evidence can also be found in tree rings, ocean sediments, coral reefs, and layers of sedimentary rocks. This ancient evidence reveals that current warming is occurring roughly ten times faster than the average rate of warming during recovery from the ice ages. Carbon dioxide from human activity is increasing more than 250 times faster than it did from natural sources after the last Ice Age.
The Planet's average temperature has risen about 2.05 degrees Fahrenheit since the late 19th century, a change driven by increased carbon dioxide and other human-made emissions into the atmosphere. Most of the warming has occurred in the past 40 years, with the six warmest years on record taking place since 2014. Not only was 2016 the warmest year on record, but eight months out of that year — from January through September, with the exception of June — were the warmest on record for those respective months. 2020 was an extremely close second. The ocean has absorbed much of this increased heat, with the top 300 feet of ocean showing warming of more than 0.6 degrees Fahrenheit since 1969. Earth stores 90% of the extra energy in the ocean. As our oceans heat up, the expand thermally which in turn has caused much of the sea level rise we have seen to date.
The Greenland and Antarctic ice sheets have decreased in mass. Data from NASA's Gravity Recovery and Climate Experiment show Greenland lost an average of 279 billion tons of ice per year between 1993 and 2019, while Antarctica lost about 148 billion tons of ice per year. And glaciers are retreating almost everywhere around the world — including in the Alps, Himalayas, Andes, Rockies, Alaska, and Africa. Satellite observations reveal that the amount of spring snow cover in the Northern Hemisphere has decreased over the past five decades and the snow is melting earlier. Both the extent and thickness of Arctic sea ice has declined rapidly over the last several decades.
Global sea level rose about 8 inches (20 centimeters) in the last century. The rate in the last two decades, however, is nearly double that of the last century and accelerating slightly every year. Since the beginning of the Industrial Revolution, the acidity of surface ocean waters has increased by about 30%. This increase is the result of humans emitting more carbon dioxide into the atmosphere and hence more being absorbed into the ocean. Ocean acidification is already impacting many ocean species, especially organisms like oysters and corals that make hard shells and skeletons by combining calcium and carbonate from seawater - the shells of some animals are already dissolving in the more acidic seawater. The ability of some fish, like clownfish, to detect predators is decreased in more acidic waters.Reef-building corals make their own homes from calcium carbonate, forming complex reefs that house the coral animals themselves and provide habitat for many other organisms. Acidification may limit coral grow by corroding pre-existing coral skeletons while simultaneously slowing the growth of new ones, and the weaker reefs that result will be more vulnerable to erosion. This erosion will come not only from storm waves, but also from animals that drill into or eat coral. A recent study predicts that by roughly 2080 ocean conditions will be so acidic that even otherwise healthy coral reefs will be eroding more quickly than they can rebuild.
And finally, the number of record high temperature events in the United States (and around the World) has been increasing, while the number of record low temperature events has been decreasing since 1950. The U.S. (and again, around the World) has also witnessed increasing numbers of intense rainfall events.
Learn More
Watch this video on impacts today of sea level rise.
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Watch this video on warming in the Arctic
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Watch this video on the melting of the Mendenhall Glacier in Alaska
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Watch this video on extreme weather events and climate change
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