With the scale of ecology now understood, we can grasp the wide scope of our subject
matter.
So now let’s dig into some details.
To understand ecology, we need to understand climate.
You’ve probably heard the terms climate and weather thrown around, as though they’re
interchangeable.
So, what’s the difference?
Think about your personality versus your mood.
Your mood can change from day to day – that’s like weather.
But your personality is the sum of all those moods over the course of many years, which
describes something more fundamental about you – that’s like climate.
Scientists use something called climate “normals”, which are defined as averages of the weather
in a certain region for a certain timeframe.
Often this is over the course of at least a decade, and up to 30 years.
In early May 2021, the National Oceanic and Atmospheric Association, or NOAA, put out
new climate “normals” for the United States over the timescale of 1991 to 2020, with the
previous normals covering 1981 to 2010.
In just this short timeframe, there have been significant changes in temperature and precipitation
in the United States.
Overall, it’s gotten warmer and wetter.
Understanding climate is important in the realm of ecology given the sensitivity of
species and populations to any changes.
Previously we mentioned that organisms have specific environmental ranges that limit them
to specific geographical regions.
We will discuss human-induced climate change later, but we must recognize that natural
climate change also exists.
Throughout geological time, climate has changed naturally, prior to human existence and influence.
The natural variability and fluctuations of the climate system have always been a part
of Earth’s history.
Natural climate changes can occur due to external variability, like changes in the Earth’s
orbit or variations in solar intensity, but also due to internal factors, like volcanic
eruptions, or ocean circulation.
Let’s look at each of these factors now.
First let’s discuss a familiar external factor responsible for natural climate change,
orbital variations.
The Earth revolves around the Sun in an elliptical orbit.
However, its shape can vary slightly over time – sometimes becoming more circular
and sometimes becoming more like an oval.
This changes how much sunlight we receive on Earth simply because there are times of
year when the planet is closer to the Sun than other times.
When Earth is closer to the sun, it receives more solar radiation.
The point at which Earth passes closest to the Sun is called the perihelion.
The point furthest from the sun is the aphelion.
An easy way to remember this is to think of “a” as in away, therefore the “aphelion”
is when the Earth is furthest away from the Sun.
The Earth itself is also slightly tilted in terms of its rotational axis with respect
to the ecliptic, or the plane of the solar system.
The amount of this tilt changes with time, as does the direction of tilt – either towards
or away from the Sun.
These shifts and wobbles in Earth’s orbit can trigger changes in climate, such as the
beginning and end of ice ages, which are geologic periods during which thick ice sheets cover
vast areas of land.
To provide some context, the last ice age ended about 12,000 years ago and the next
cooling cycle may begin in about 30,000 years if we were to make a prediction based on orbital
cycles alone.
Orbital changes are so gradual that they’re only noticeable over thousands of years – not
decades or centuries.
The collective effects of changes in the Earth’s orbital movements upon its climate are called
Milankovitch cycles, named after Serbian astronomer and geophysicist, Milutin Milankovic.
There are several parameters that are relevant here.
Eccentricity is the technical term used to describe how much the Earth’s orbit around
the Sun varies from more circular to more elliptical.
This is due to the Sun and the gravitational attraction exerted by other planets.
There are two periodicities, or cycle lengths, that have been identified: one cycle is an
average of 100,000 years, and another, longer, cycle is about 413,000 years.
In periods of high eccentricity, radiation exposure on Earth can fluctuate more wildly
between perihelion and aphelion.
Fluctuations are milder in times of low eccentricity, when Earth’s orbit is more circular, leading
to more stability in the climate.
Obliquity is the technical term used to describe the tilt of the Earth.
Currently, the Earth is tilted at 23.5 degrees to the vertical.
This value fluctuates from 22.1 to 24.5 degrees, which can lead to small but climatically important
changes in the seasons.
For example, more tilt means warmer summers and colder winters; less tilt means cooler
summers and milder winters.
The periodicity of this change is on the order of 41,000 years.
Finally, precession is the technical term for the motion of Earth’s axis of rotation.
It does not point in a fixed direction, but rather moves in a clockwise direction in space,
like the spinning of a wobbling top.
Of the three, precession has the shortest periodicity: 19,000 – 23,000 years.
Solar radiation received at low latitudes on Earth is principally affected by variations
in the cumulative effect of eccentricity and precession, whereas the higher latitudes are
mainly affected by changes in obliquity.
Since the Earth is tilted in its orbit, the regions of Earth do not receive the same amount
of energy – more energy is received at the equator than at the poles.
This difference can be exaggerated at different times.
These slow changes in the Earth’s orbit lead to small but climatically important changes
in the strength of the seasons over tens of thousands of years.
Additionally, other climate feedbacks in the internal climate system can amplify these
small orbital changes.
In the shift between glacial and interglacial periods on Earth, this is most related to
the intensity of summers in the Northern Hemisphere, which has more land than water.
Land warms faster than water, so this hemisphere is typically more sensitive to changes in
solar radiation.
For example, when summers are mild, enough snow and ice remain throughout the season,
maintaining glaciers, and allowing them to grow larger in the winter.
When summers are hotter, more ice melts in the summer than can be replenished in the
winter, and ice growth is hindered.
A “perfect orbital storm”, so to speak, for global warming would require Earth’s
orbit at its highest eccentricity, Earth’s axial obliquity at its highest, and the Northern
Hemisphere in perihelion at summer solstice.
Rather, in our current configuration, Earth’s Northern Hemisphere currently experiences
its summer in aphelion, the planet’s obliquity is currently on the lower end, and the orbit
is fairly circular.
In fact, the current organization of Earth’s orbital features should result in cooler temperatures,
but instead, the average temperature of the planet is on the rise.
Thus, we cannot attribute current climate trends to orbital parameters.
Let’s also take a look at the Sun itself.
The Sun is the source of energy for the Earth’s climate system.
Although the Sun’s energy output appears constant from an everyday point of view, small
changes over an extended period of time can lead to climate changes.
Scientific studies demonstrate that solar variations have played a role in past climate
changes.
For instance, a decrease in solar activity was thought to have triggered the Little Ice
Age, a cool period in the Northern Hemisphere between 1650 and 1850, when Greenland was
largely cut off by ice from 1410 to the 1720s, and glaciers advanced in the Alps.
What about current global warming?
The Sun’s energy fluctuates on a cycle that’s 11 years long – and the energy changes only
by about 0.1% during each cycle.
Since 1750, the average amount of energy coming from the Sun either remained constant or increased
slightly.
Since 1870, solar fluctuations have contributed a maximum of only a tenth of a degree Celsius
to global temperature changes.
Scientific observations of solar activity, in fact, have shown a slight cooling trend
since 1960, with the last few decades showing the sun and climate trends moving in opposite
directions, so changes in the sun have actually contributed a slight cooling influence in
recent decades.
It is clear therefore that the warming we are seeing on the Earth is not due to solar
activity.
If that were the case, scientists would expect to see warmer temperatures in all layers of
the atmosphere as well.
Yet, they have observed cooling in the upper atmosphere, and warming at the surface and
in the lower parts of the atmosphere, due to greenhouse gases like carbon dioxide, methane,
and water vapor, which get trapped in these lower layers.
Greenhouse gases are named as such because they can capture heat.
If you want to feel this for yourself, walk into a greenhouse on a hot day!
The roof of a greenhouse acts as a physical barrier, keeping heat in, just as carbon dioxide
and other greenhouse gases act as a barrier to longwave radiation attempting to leave
the atmosphere.
Climate models that include solar changes cannot reproduce the last century’s observed
temperature trends without including a rise in greenhouse gases.
With orbital variations and the sun covered, let’s move forward and explore other factors
that have an impact
on climate.