4: Global Climates and Ecosystems

In the previous chapter we discussed climate only briefly, focusing on some of the causes and consequence of global warming. This chapter goes into more detail on how we classify different climates around the world, what those climates are like, and the characteristic communities of plants and animals we see associated with major climate types.

1. Climographs: Visually Representing Climate

Reviewing the last chapter, climate is the average of  weather over a long period of time, usually 30 years or more. When constructing this average, scientists don't ignore seasonal variations in the data, as it's important to know if January is much colder than July or if there isn't much difference. Two cities used as examples in the previous chapter, Wichita and San Francisco, have average annual temperatures within about 2 degrees F of each other. But there are big differences between the climates of the two places!

So, to describe a climate, we should not only have a way of describing average annual rainfall and average temperatures, but also the seasonal variation in those. It's possible to write them out, but it's even easier to compare multiple climates by using a graphic representation of climate called a climograph.

Before introducing the most common climates of the world, let's take a look at a climograph for Seattle, Washington:

Just glancing at the climograph we can tell a number of things about Seattle's climate:

  • Temperatures are moderate, with summers not very hot and winters not very cold. 
  • No month is on average below freezing, meaning that snowfall is possible but relatively rare.
  • Rainfall is abundant, with a summer minimum but generally plenty of precipitation year-round. 
We'll use climographs like this to represent various climates around the world to understand how different climates can be.

2. A map of world climates

There are a number of different ways to classify Earth's surface into different climate regions. In Chapter 1, a formal region is defined as having at least one common characteristic everywhere inside the region. Climate regions are formally defined by their precipitation and temperature characteristics, but keep in mind that the lines you see dividing the world into climate regions rarely see abrupt transitions. Rather, the boundary of a climate zone marks where the conditions change however they are defined by the map maker. For example, a desert region might be defined as "areas with an annual average of less than 10 inches of precipitation." (Note that desert regions are actually defined through a more complex formula than this to more accurately reflect the effects of temperature on moisture availability). But at the border of that desert region, we could move a few hundred feet beyond the clearly defined desert into an area that receives an average of 10.1 inches of rain...not much wetter. So keep in mind that these boundaries are usually zones of transition between one climate and the other; even though the boundary is clearly defined, climates exist on a continuum of precipitation and temperature between each other. 

This book uses a relatively simple set of climate regions for ease of understanding: 

World climate regions. Map from Wikimedia Commons, Waitak at the English language Wikipedia [GFDL (www.gnu.org/copyleft/fdl.html) or CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/)]

Let's start with some important observations about the map:
  • The climates are listed on the map starting at the Equator and moving poleward (both north and south from the Equator); Tropical climates first, then Dry climates, then Moderate climates, followed by Continental and Polar. 
  • Continental climates occur only in the Northern Hemisphere. The Southern Hemisphere does not have large enough land masses at high latitudes for continental climates to exist.
  • There are five major climate types but each has at least two sub-types. These climate sub-types are distinctively different from one another, but have at least one thing in common with the major climate type.
The next sections describe each of the climates in the maps as well as their biomes. A biome is a distinctive collection of plants, animals, and their characteristic habitat. There are often multiple biomes for a climate region but we'll look at least one example  biome for each climate we consider.

3. Tropical climates

The unifying characteristic of tropical climates is that they are quite warm year-round, with little annual variation in temperature. There are not summer and winter seasons near the Equator, as day length does not vary by much (at the Equator it's nearly always 12 hours of daylight and 12 hours of darkness year-round).

Remember that the climograph temperatures are showing the 24-hour average temperature (not the day's highs or lows) for the entire month over 30 years, so average temperatures in the 70s is quite warm!

We'll look at two tropical climates in particular: the tropical wet climate, also known as the tropical rainforest climate, and the tropical wet and dry climate, also known as the tropical savannah climate.

The tropical wet climate and tropical rainforest biome

As already reviewed, tropical climates have warm temperatures year-round. The distinguishing feature of the tropical wet climate is its abundant year-round precipitation. Here is a climograph for Nuevo Rocafuerte, Ecuador, in the Amazon River basin: 

Compare this climograph to one for Seattle. You should be able to appreciate that the average temperature is much warmer. In fact, every month in Nuevo Rocafuerte is warmer than Seattle's warmest month. Likewise, the driest month, December, has a little more rainfall on average than Seattle's wettest month. 

With warm temperatures year-round and abundant water, tropical wet climates have great conditions for plant growth. The characteristic biome of the tropical wet climate is the tropical rainforest. The tropical rainforest biome is so often associated with this climate that sometimes people refer to the climate by the same name.

Tropical rainforests feature plant growth on every surface. Multiple layers of plants compete for the sun's energy, with tall trees reaching for the sky and forming an upper "canopy" that blocks some of the light. Lower levels of plants have broad, dark green leaves to absorb as much remaining light as they can. 

The rainforest of the upper Amazon River basin, in eastern Ecuador, displays multiple levels of lush vegetation. 2007 photo by Tim Scharks.

Tropical wet and dry climate and savannah biome

The cousin of the tropical wet climate is a drier version that receives ample precipitation during part of the year but also has at least one dry season. During the dry season temperatures remain high and there is not enough precipitation for plants to grow well, so grasses and shrubs go dormant. Trees are either drought tolerant or have long tap roots to access water deep underground.

Examine this climograph for Arusha, Tanzania, East Africa: 

The average temperature is lower than we might expect for a tropical climate because of Arusha's high elevation, about 4,500 feet above sea level. Still, though, temperatures are high and there is little variation through the year. However, rainfall has a pronounced peak in the three months of March, April, and May; November and December receive enough rain to be considered "wet" but there is a short dry season from January to February and a long dry season from July to October.

The characteristic biome of the tropical wet and dry climate is the savannah. Savannahs can range from grasslands to sparse woodlands of thorny Acacia trees. The savannah biome in Africa is famous for supporting large numbers of grazing animals as well as the predators that prey upon them.

Two pictures of giraffes taken in Lake Manyara National Park, Tanzania. The image on the left was taken in early January at the end of the "short rains." The image on the right was taken mid-August, in the middle of northern Tanzania's long dry season. Photo at left 2003, photo at right 2012 by Tim Scharks.

4. Dry Climates

The dry climates are defined by a lack of moisture for a large part or all of the year. Our classification scheme has two subtypes of the dry climates: arid and semiarid. Different criteria may be used to distinguish between these types but a general rule is that semiarid climates have enough moisture for grasses and hardy shrubs to grow, but not many other plants. Semiarid climates may receive enough seasonal rainfall that there is some runoff in the form of small streams and rivers, but these can go dry during the dry season and there are not many rivers.

Arid climates, in contrast, receive so little moisture that almost no vegetation can grow. These climates are dominated by bare ground. The only plants that survive are extremely hardy and have special adaptations to survive extremely long drought conditions. These extreme drought-tolerant plants are called xerophytes.

Let's take a look at two climographs for the dry climates side-by-side. The first is from Riyadh, a hot desert and a good example of the arid type; the second climograph is from Elko, Nevada, a semiarid climate. The comparison risks confusion since one climate is much cooler than the other; however, temperature is not the defining characteristic for dry climates, rather it is moisture availability. Elko receives only 9.5 inches of precipitation each year, but because of cooler temperatures (two winter months average below freezing), the precipitation does not evaporate as quickly.

The arid climate is associated with a desert biome. The photo below shows an area dominated by wind-drifted sand, but there are areas in this biome where sparse plant life survives, especially in areas where water can accumulate when it runs off of rocks and the ground is shaded for part of the day.
Dry arid landscape near Riyadh, Saudi Arabia. Photo by Flickr user ActiveSteve, used under Creative Commons CC BY-ND 2.0 license. Source: https://www.flickr.com/photos/activesteve/5208459754/
Dry semiarid landscapes like those around Elko, Nevada, are home to the sagebrush steppe biome. This biome is dominated by sagebrush, a low, hardy shrub, and varieties of bunch grasses. Another common biome for semiarid climates is the grassland biome, where grasses dominate the plant growth.

"Sagebrush-Steppe in northeastern Nevada along US 93" by Famartin - Own work. Licensed under Creative Commons CC BY-SA 3.0 via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Sagebrush-Steppe_in_northeastern_Nevada_along_US_93.jpg

5. Moderate Climates

Moderate climates are defined by their temperature characteristics: They do not have any month with temperatures below freezing, and average temperatures are generally mild, though the humid subtropical subdivision has hot summers. For this reason the moderate climates are also known as temperate climates.

Humid subtropical climate and deciduous forest biome

The humid subtropical climate occurs just north and south of the tropics. Like many dry climates, the humid subtropical climate has warm temperatures, especially in summer. However, the "humid" designation in the name of this climate indicates that there is ample precipitation year-round. Our representative climograph is from New Orleans, Louisiana, in the United States: 

Much of the eastern United States has the deciduous forest biome, characterised by trees with flat, broad leaves. In locations like New Orleans winter temperatures do not drop very low so many deciduous trees do not lose their leaves seasonally, or lose their leaves as new leaves are growing in the spring, as is the case for southern live oak trees. Farther north, especially in continental climates, the deciduous forest loses its leaves and becomes dormant during the winter.

Marine west coast climate and the temperate rain forest biome 

We've already looked at a climograph for Seattle, Washington, but now we can place it in our range of climates. Seattle has a marine west coast climate, characterized by mild temperatures year-round and ample precipitation with a low (but not full drought conditions) in July.

Seattle does not receive as much total precipitation as areas where orographic precipitation has a greater effect. It's in more mountainous areas that we see the full expression of the biome that occurs in sync with the marine west coast climate, the temperate rain forest biome. Temperate rain forests are dominated by coniferous, needle-leaved evergreen trees like Douglas Fir and Western Red Cedar. The abundant precipitation promotes lush vegetation including lots of moss and ferns on the forest floor.

Backpackers hike through a temperate rain forest on the west coast of Vancouver Island, British Columbia. 2004 photo by Tim Scharks.

Mediterranean climate and Mediterranean shrubland biome

The Mediterranean climate is one of the smallest climate zones on Earth, so it's interesting that it has its own division. It merits its own category because of its unique characteristics, including its distinctive biome, and its long association with European history. Mediterranean climates are considered the ideal place for the cultivation of some specific crops we associate with the cuisine of places like Italy and Greece: grapes, olives, and wheat, to name a few.

The climate receives its name from the shores of the Mediterranean sea but we also see it in four other places on Earth: central coastal California, central coastal Chile, far southern coastal South Africa, and the southwest coast of Australia. In every case the sea serves as an important source of moisture during the winter. In summer, however, the Mediterranean climate experiences a pronounced dry season. This dry season is the characteristic that marks the Mediterranean climate as different from the other moderate climates.

Mediterranean climates are extensively settled by humans so it may be difficult to observe the natural biome in this climate region. However, many hillsides still exhibit the Mediterranean shrubland biome, as this picture from above the eastern side of San Francisco Bay shows: 

Sibley Regional Park, Berkeley Hills. Photo by Flickr user Jono Hey, used under Creative Commons CC BY-NC-SA 2.0 license. Source: https://www.flickr.com/photos/palojono/3838435568
This biome features trees, generally short in height (thus "shrubland"), hardy shrubs, and sometimes extensive patches of grass. All species are drought tolerant. The biotic landscape of the Mediterranean shrubland in its natural state is strongly influenced by fire. Regular fires maintain clearings and a park-like appearance.

6. Continental climates 

Humid continental climate and the deciduous forest biome

Like the humid subtropical climate, the humid continental climate has year-round precipitation. The difference is that the humid continental climate has at least one month colder than freezing, with warm to hot temperatures in summer. The word "continental" in this climate's name refers to the influence of continentality-this climate tends to occur in the center of large land masses or where the prevailing wind moves from the center of the land mass towards the coast. For an example of a humid continental climate, examine Chicago, Illinois, USA: 

Three months average below freezing, meaning snow is a regular occurrence in this climate. Temperatures are high in the summer months so there is a large range in annual temperature.

Many places with humid continental climates have a deciduous forest biome like that of the humid subtropical climate. Unlike places like New Orleans, though, deciduous forests in humid continental climates drop their leaves in winter.

The deciduous forest biome loses its leaves for the winter months. Both images public domain from pixabay.com.

Subarctic climates and the boreal forest biome

Meaning "just outside the Arctic", the subarctic climate occurs at high latitudes in the Northern Hemisphere. Like the humid continental climate, the subarctic climate had a large annual temperature range. But temperatures overall are colder, to the point where summers are still not very hot on average and winters are long and cold. A representative location for the subarctic climate is Whitehorse, a city in Canada's Yukon Territory:
Winter is long and cold, with temperatures averaging below freezing for five months of the year. Even though precipitation appears very low, remember that precipitation falls as snow for nearly half the year. This fact, coupled with cool average temperatures in summer, means that not much evaporation takes place, so there is enough moisture for abundant plant growth. The factor limiting plant growth is the cold, long winter.

The subarctic across North America and Eurasia is dominated by the boreal forest biome. Boreal is another word for "northern." Boreal forests contain hardy deciduous and evergreen needle-leaf trees. Trees do not grow very large compared to other forest biomes.

A canoe is carried through a boreal forest on Alaska's Kenai peninsula. The white trunks are birch trees, the darker needle-leaf trees are black spruce. 2003 photo from Tim Scharks collection.

7. Polar climates and the tundra biome

North of the subarctic climate, average temperatures drop even lower. The tundra climate and tundra biome are synonymous because the tundra climate is defined by a growing season too short and cold for trees to grow. Let's look at the tundra climate of Barrow, Alaska:

Like the subarctic climate, even though annual precipitation appears low, temperatures are so cold that little evaporation takes place. Thus, the tundra biome is frequently waterlogged. Tundra is characterized by low plants like grasses, sedges, lichens, and mosses that can tolerate the extreme cold and short growing season of the far north.

Tundra near Point Hope, Alaska in late fall. Photo by Flickr user Josh Kellogg, used under Creative Commons CC BY-NC-ND 2.0 license. Source: https://www.flickr.com/photos/kelloggphotography/3112675665/

Finally, at extreme northern and southern latitudes where annual temperatures never reach above freezing, ice sheet climates exist. These climates are confined to Greenland and Antarctica.

For an extreme example, let's take a look at the coldest place on Earth, Vostok Station, Antarctica:

Temperatures are too cold for much precipitation to fall. Vostok averages only 0.2 inches of water equivalent per year. The standard temperature range is reversed because the Antarctic summer starts in December and winter starts in June. There is no biome associated with this climate, as so few plants and animals survive here.

Much of Greenland has an ice sheet climate. In this climate cold temperatures lead to the accumulation of large masses of ice, which flow downhill as glaciers. 2003 photo by Tim Scharks. 

8. Highland climates: a special case

Climate regions are given the "highland" designation when altitude overwhelms the other influences on climate. Mountainous areas display dramatic differences in climate within just a few miles because of decreases in temperature with elevation. Furthermore, orographic precipitation can create dramatic wet side/dry side effects in precipitation. The highland climate designation is used because one climate classification does not fit well. Biomes are also complex and can vary dramatically within just a few miles. The effect of altitude on climate to create many different zones of biomes with changing elevation is called altitudinal zonation.  As mentioned in Chapter 3, even at the Equator high enough mountains feature glaciers on their summits. In these cases altitude has effectively made an ice sheet climate possible at the equator. 

Edited October 29, 2014