What is human-induced climate change?

The state of the Earth's climate is a result of complex interactions between the atmosphere, the oceans, ice (on land and water), the land surface, and the underlying Earth. Climate has always varied, and will continue to display natural variability on many timescales. Some of these variations are associated with volcanic eruptions, changes in the Earth's orbit, or changes in the energy output of the Sun. Others are due to natural processes within the climate system itself. Since the industrial revolution of the early 19th century, human activities have also markedly influenced the climate. This well-documented human-induced change is large and very rapid in comparison to past changes in the Earth's climate.

The main human influences on the climate system result from changes in the composition of the atmosphere, primarily through the emission of greenhouse gases. The most important of these is carbon dioxide (CO2). On timescales of millions of years and longer, the concentration of CO2 in the atmosphere is controlled by geological processes. Carbon naturally accumulates in geological reservoirs including hydrocarbons such as petroleum, natural gas, and coal. The extraction of hydrocarbons from these geological reservoirs and their combustion has increased atmospheric concentrations of CO2, driving human-induced climate change. Deforestation over the past two centuries has also contributed significantly to this increase. The atmospheric CO2 concentration now exceeds by about one third its highest concentration at any time in the past 850,000 years. It is at a level not seen since the Pliocene epoch more than two and one half million years ago, a time predating not only human civilization but humanity itself.

Although CO2 is a natural constituent of the Earth's atmosphere, and helps keep the Earth's surface warm, human activities have increased its concentration by over 40% since the beginning of the industrial revolution. The concentrations of other greenhouse gases (such as methane and nitrous oxide) that are less abundant in the atmosphere have also been increasing as a result of human activity. As the concentration of these greenhouse gases increase, the warming effect is enhanced.

The oceans are a major reservoir of CO2. Roughly 25-30% of the CO2 emitted into the atmosphere by humans has been taken up by the ocean making it more acidic – the process of ocean acidification. As seawater becomes increasingly acidic, it becomes more corrosive to the shells and exoskeletons of a wide variety of marine organisms. Together with ocean warming, acidification results in enhanced stresses on ocean ecosystems.

As a result of increases in greenhouse gases, the atmosphere and ocean are warming – and associated with this warming are changes in precipitation, winds, sea level, and snow and ice cover. Together with ocean acidification, these changes make up human-induced climate change. Past and future climate changes: The global picture.

Direct observations demonstrate that the Earth's surface has been warming for the past century. Although natural climate variability produces rises and falls in the temperature record, there is an overall trend toward global-scale surface warming that is outside the envelope of natural variability. The first decade of the 21st century has been the warmest in the instrumental record extending back to 1850. No single year since 1985 has recorded a global mean temperature below the average of the period 1960-2010. There is strong evidence that the frequency of heavy precipitation events in North America has been increasing. Worldwide, almost all glaciers have been retreating and the Greenland and Antarctic ice sheets have been losing mass. Both the extent and volume of summertime Arctic Ocean sea ice have reduced dramatically in recent decades, with unprecedented losses in 2007 and 2012.

An aspect of the global temperature record that has received considerable attention is the apparent “hiatus” in global-mean surface air temperature increase over the past decade. Variability in the rate of warming is expected. There have been periods over the last 150 years, during which we have reliable data, in which there have been similar plateaus and indeed even short declines. In looking at climate change we must focus on the long-term trend; this has shown an unequivocal increase of 0.9oC since 1901.

These changes in the Earth's climate are all consistent with our understanding of the basic physical and chemical processes by which it is governed. By expressing these principles mathematically, climate models can make projections of the climate response to emissions of greenhouse gases and other climate change drivers.

These models project warming at the surface and in the lower atmosphere across the globe in response to increases in atmospheric greenhouse gases. While some locations may experience temporary cooling due to changes in the circulation of the ocean and atmosphere, the great majority of locations will warm. The warming is often measured in terms of “climate sensitivity” - the increase in long-term surface temperature for a doubling of atmospheric CO2. A range of values between 1.5 and 4.5oC is considered to be a robust result which has not changed dramatically as our understanding of the climate system has improved. As with the instrumental record, climate models display “natural” variability in the rate of temperature increase resulting from greenhouse gas emissions. Together with regional-scale models, climate models further project:

• longer and more intense heat waves
• increases in extreme precipitation
• an accelerated loss of summertime Arctic Ocean sea ice, with the possibility of a seasonally nearly ice-free Arctic Ocean by the mid 21st century (or earlier)
• an increased rate of melting of glaciers and the Greenland and Antarctic ice sheets
• rises in global sea level due to a combination of the melting of ice on land and the thermal expansion of seawater as the ocean warms
• increased acidification of seawater

These changes will have ecological and socio-economic impacts. Specific risks to humanity include flooding due to extreme precipitation and rising sea level, as well as losses and shifts in agriculturally productive land due to warming and changes in the water cycle. Ecosystem impacts will involve changes in the habitats for some organisms and the loss of habitat for others. While organisms have adapted to past climate changes, those changes occurred much more slowly than those of the present human-driven change. It is expected that these projected climate changes will result in substantial loss of biodiversity as well as a redistribution of species.

There are uncertainties in these projections. The climate system is complex and involves processes on many different space and time scales. Since climate models require some simplifications and approximations, and some of the processes they represent are imperfectly understood, quantitative projections often differ between models using the same future changes of greenhouse gas concentrations. Beyond this, there are many scientific challenges in understanding the interplay of different components of the climate system. Uncertainty regarding the value of the climate sensitivity and the fact that the recent “hiatus” in global-mean surface air temperature increase is not captured by models are questions that require ongoing study.

There is an ongoing global effort to better understand, and where possible reduce, these uncertainties. The global state of the art in understanding of climate change is summarized in reports by the Intergovernmental Panel on Climate Change (IPCC), whose most recent Fifth Assessment Report is being released in 2013 and 2014.

Past and future climate changes: The Canadian picture.

The impacts of climate change will be felt differently around the world. For Canada, these impacts are largely determined by its location in the northern middle and high latitudes, and by its long coastline. The years 2001-2010 were the warmest decade in Canada since measurements began, and the anomalous warmth was greater than elsewhere in North America. Climate models project that human-induced warming will be intensified towards high northern latitudes so projected warming over Canada is greater than the global average. Although this warming may have some beneficial aspects (such as longer growing seasons), other effects will be negative and pose a challenge to adaptation. For example, expected changes in the seasonality of the water cycle may result in more wintertime precipitation in Western Canada falling as rain rather than snow, potentially reducing the water available for downstream agriculture in the spring and summer. Longer and more intense heat waves, as well as more intense precipitation extremes, are expected. A seasonally ice-free Arctic Ocean will provide opportunities for transport through this area, but will profoundly affect local ecosystems and human communities. Rising sea level will increase coastal erosion and threaten coastal infrastructure; these changes will be particularly pronounced in the Arctic because of retreating sea ice. Melting permafrost will also threaten northern infrastructure, and the associated release of stored carbon is expected to amplify the changes in atmospheric composition resulting from the combustion of fossil fuels. Ocean acidification and warming stresses on ocean ecosystems are expected to be particularly acute in the Arctic. Human-induced climate change is expected to pose significant challenges for Canadians through the 21st century and beyond.

Responding and adapting to climate change: a challenge for Canadians.

Even if the human-induced emission of greenhouse gases into the atmosphere were to cease today, past emissions have committed the world to long-term changes in climate. Carbon dioxide emitted from the combustion of fossil fuels will remain in the atmosphere for centuries to millennia, and the slow ocean response to atmospheric warming will cause the climate change to persist even longer. Further CO2 emissions will lead to greater human-induced change in proportion to total cumulative emissions. Meaningful interventions to mitigate climate change require a reduction in emissions. To avoid societally, economically, and ecologically disruptive changes to the Earth's climate, we will have little choice but to leave much of the unextracted fossil fuel carbon in the ground. So-called geoengineering solutions have been proposed to counteract the effect of greenhouse gas emissions. These remain speculative, however, and they may have substantial unintended consequences. Furthermore, many of these proposed solutions do not address the issue of ocean acidification.

Although Canada represents only 0.5% of the global population, it contributes 1.8% of global CO2 emissions (as of 2008) – resulting in one of the highest per capita emission rates in the world. The more fossil fuels are burned, the greater the climate change. The urgent challenges for the global community, and Canadians in particular, are to learn how to adapt to the climate changes to which we are already committed and to develop effective and just responses to avoid further damaging climate change impacts for both present and future generations.