Global Warming

The carbon cycle is a natural and integral part of life on Earth. A single carbon atom stored in a blade of grass may shift to the body of an animal that eats the grass.

What is the greenhouse effect?

The greenhouse effect is one of the main factors determining the temperature of a planet. It’s the phenomenon by which certain gases ‘so-called greenhouse gases’ in the atmosphere trap heat that would otherwise escape to space, thereby keeping the planet warm.

The greenhouse effect is not a man-made phenomenon. The Earth’s atmosphere has always contained greenhouse gases, such as CO2, and they have always caused warming. If there was no greenhouse effect, the planet would be uninhabitably cold ‘more than 30°C colder than the hospitable current average of 15°C.

However, humans are changing the strength of the greenhouse effect by increasing the proportion of greenhouse gases in the air. For example, CO2 concentration in the atmosphere has increased from around 315 parts per million (ppm) to 387 ppm since 1959.

What is the carbon cycle?

Just as the “water cycle” describes the transfer of water molecules between rivers, oceans, land and the atmosphere, the “carbon cycle” refers to the flow of carbon through different parts of the Earth system ‘including the air and the bodies of plants and animals.

The carbon cycle is a natural and integral part of life on Earth. A single carbon atom stored in a blade of grass may shift to the body of an animal that eats the grass. When the animal dies, its body might rot, and the carbon atom could join with oxygen to form CO2 in the air. From there, it might be taken in by a tree in the process of photosynthesis and used as a building block in a branch or trunk, or absorbed by the ocean. And so on.

Although the basic flows of the carbon cycle haven’t significantly changed, in the last century or so humans have increased the amount of CO2 in the air by taking carbon that has been locked up in the ground for millions of years ‘in the form of oil, coal and gas’ and releasing it into the atmosphere by burning fossil fuels. There is strong evidence that this has led to global warming.

Are hurricanes getting worse because of global warming?

There’s tremendous variation in hurricane activity over time and from place to place. Various studies published since 2005 indicate that the number and/or strength of hurricanes have increased in various regions, especially since the 1970s. However, it’s likely that some hurricanes at sea went unnoticed in the days before satellites and hurricane-hunter aircraft, and that complicates the assessment. There’s no doubt, though, that hurricane activity has stepped up since the mid-1990s in the North Atlantic, where ocean temperatures have risen through long-term warming and an apparent multidecadal cycle in Atlantic currents. The tropics are part of a global trend toward ocean warming that goes hand in hand with atmospheric warming, and warm oceans provide the energy to drive hurricanes.

Trends aside, a catastrophic storm can strike in any year, and it’s impossible to tie any single hurricane or other weather event directly to global warming.

How do trees and forests relate to climate change?

Deforestation, and especially the destruction of rainforests, is a hugely significant contributor to climate change. Scientists estimate that forest loss and other changes to the use of land account for around 23% of current man-made CO2 emissions ‘which equates to 17% of the 100-year warming impact of all current greenhouse-gas emissions.

As children are taught at school, trees and other plants absorb CO2 from the air as they grow. Using energy from the sun, they turn the carbon captured from the CO2 molecules into building blocks for their trunks, branches and foliage. This is all part of the carbon cycle.

A mature forest doesn’t necessarily absorb much more CO2 that it releases, however, because when each tree dies and either rots down, or is burned, much of its stored carbon is released once again. In the context of climate change, the most important thing about mature forests is not that they reduce the amount of CO2 in the air but that they are huge reservoirs of stored carbon. If such a forest is burned or cleared then much of that carbon is released back into the atmosphere, adding to atmospheric CO2 levels.

Of course, the same process also works in reverse. If trees are planted where previously there weren’t any, they will on soak up CO2 as they grow, reducing the amount of greenhouse gas in the atmosphere. It is thought that trees, plants and other land-based “carbon sinks” currently soak up more than a quarter of all the CO2 that humans add to the air each year ‘though that figure could change as the planet warms.

Is the sun causing global warming?

Over the very long term, variations in Earth’s orbit that shape where and when sunlight reaches the planet are the main cause of ice ages, but this doesn’t apply to our current situation. It’s true that the Sun has produced more sunspots in recent decades than it did in the early 1800s. However, this mainly reflects an increase in the ultraviolet range of sunlight, which is only a tiny part of the solar spectrum.

In fact, the total solar energy reaching Earth changes very little over time. Across the 11-year solar cycle, it varies by less than 0.1%, and even across the period since the little ice age chill of 1750, solar output climbed no more than about 0.12%, according to the 2007 report from the Intergovernmental Panel on Climate Change. And a recent analysis of solar trends suggests that over the last few decades, the sun has actually contributed a slight cooling effect, rather than accounting for any of the observed warming.