At the start of the 20th century, Swedish scientist Svante Arrhenius put forward the idea that human emissions of carbon dioxide would eventually raise temperatures. He didn't see this as a particularly bad thing, and most scientists at the time were skeptical that humans could burn fossil fuels fast enough to have a noticeable impact.
Only relatively recently have scientists confirmed with confidence that mankind could influence the global temperature. Before scientists could understand the influence of humans, data had to be gathered from around the world, technology had to be developed to analyze those data, and basic advances in physics and other disciplines had to come about. What we now know about the climate is thanks to generations of dedicated researchers.
Observing human-caused climate change is easier today because of the amount of carbon dioxide put into the climate system over the past century. The impacts of climate change are now clearly visible, affecting people and ecosystems all over the world. More cars, more factories and more power plants are changing the climate faster than was previously possible, and causing more obvious changes.
Taking the planet’s temperature
For an accurate picture of how warm the Earth is, scientists need measurements from all over because the whole planet does not heat up at the same rate. In fact, some parts might even cool down while the world as a whole heats up. Also, many temperature readings are needed over time to develop an accurate long-term picture. In order to develop a history of global temperatures, researchers have had to travel to the farthest corners of the Earth, and come up with ways to go back in time.
Some sources of past temperature data:
- Historical records: Ship’s logs, farmers’ diaries and newspaper articles. Careful evaluation can provide quantitative and qualitative data.
- Personal accounts and oral histories: Older generations of indigenous people who have always relied on nature for their survival are particularly observant of changes over the past decades.
- Direct measurements: Only go back about 300 years, and are very sparse until about 150 years ago. Also, differences in thermometer types and other variables have to be taken into account.
- Data collected by balloon and satellite: Very useful, but only available since 1979.
- Tree ring thickness: Width and density varies depending on growing conditions.
- Ocean and lake sediments: Billions of tonnes of sediments accumulate each year. The tiny preserved fossils and chemicals in layers of sediment can be used interpret past climate.
- Coral skeletons: The water temperature that the coral grew in can be determined from trace metals, oxygen and the isotopes of oxygen contained in its skeleton.
- Fossil pollen: Each plant has uniquely shaped pollen. Knowing what plants were growing at a particular time in the fossil record lets scientists make inferences about what the climate was like at the time.
- Ice cores: Over the centuries, snow falling on high mountains and on the polar ice caps packed down and became solid ice. Dust and air bubbles trapped in this ice provide valuable climate data. Air trapped in ice serves as a record of carbon dioxide concentrations across the millennia.
- • Observed melting: Rates of glacial retreat, permafrost thaw, shrinking polar ice caps and reduction in Arctic sea ice are indicators of short- and long-term climate change.
Taken together, these data produce a scientifically compelling picture of a warming world that matches with the corresponding increase in greenhouse gasses.
Predicting the climate future
Global climate models are mathematical representations of the real world’s climate. Some models are attempts by scientists to boil the complex behaviour of the climate down to comparatively simple formulas in an attempt to understand the forces at work. However, when people talk about specific predictions of long-term climate conditions, they are usually talking about general circulation models. In these models, the equations are tweaked (within reason) until the model is able to predict past and present conditions as accurately as possible when tested against actual observations of past and present conditions.
It’s impossible to know every last variable so the model will never match the real world perfectly. Therefore, scientists compensate by running each model over and over, with tiny changes to the starting conditions and other factors to get an idea of the different possible outcomes. The most likely outcome is the one that results most often.
In the end, each model predicts a range of possible outcomes. For example, the Intergovernmental Panel on Climate Change, taking into account all of the different available models, settled on a projected global temperature rise of 1.4 to 5.8 C. No one can say exactly how much the temperature will increase over the next 100 years, but with a couple of caveats it is a safe bet that it will be within this range.
Climate models cannot predict all the possible effects of feedback mechanisms, which might help stabilize the climate or cause the climate to change much faster and in unpredictable ways. Of course, it would be irresponsible to ignore the climate models and hope for the best because of these uncertainties.
These models also cannot predict human behaviour and ingenuity. We could burn more fossil fuels than expected and end up with a hotter planet than even the worst-case scenario. Or we could deploy solutions such as renewable energy and energy efficiency faster than thought possible, thereby eliminating the likelihood of the higher temperatures.