FAQ

Multiple independent lines of evidence show with high confidence that human activities are responsible for the observed rise in atmospheric carbon dioxide (CO₂) since 1750, and that this rise is inconsistent with natural sources.

The carbon cycle involves the movement of carbon between different reservoirs on Earth — the atmosphere, oceans, terrestrial biosphere, and the solid Earth, including fossil-fuel reserves. While carbon naturally moves among these reservoirs, the total amount of carbon on Earth remains essentially constant. Over the 10,000 years preceding the Industrial Era, this natural carbon cycle was roughly balanced, with atmospheric CO₂ concentrations remaining nearly stable. Since the start of the Industrial Era, CO₂ in the atmosphere has rapidly increased. Over 1750–2011, the atmospheric increase was 240 Pg ⁸⁹ (90% uncertainty range from 230 to 250 Pg) of carbon (C), as shown by air samples from ice cores and by direct measurements of atmospheric CO₂ concentrations since 1958. How do we know that this measured increase was due to human activities rather than to changes in the natural carbon cycle?

From our records, we know that humans emitted 375 Pg C (90% uncertainty range from 345 to 405 Pg C) into the atmosphere from burning fossil fuels and manufacturing cement, and we can estimate that human-induced land use change (including deforestation and reforestation) contributed a further 180 Pg C (90% uncertainty range from 100 to 260 Pg C) to the atmosphere over the period 1750–2011. Together, these human emissions totalled 555 Pg C 90% uncertainty range from 470 to 640 Pg C). Since we know that the increase in atmospheric CO₂ (240 Pg C) was less than that amount, it follows directly that the natural system must have been a net sink of carbon over this period. This is known as the “bookkeeping method,” and it is a strong piece of evidence that human emissions, rather than natural sources, are responsible for the observed increase in atmospheric CO₂. There is also direct evidence that individual natural reservoirs have acted as sinks for atmospheric carbon. For example, the measured carbon in the oceans is estimated to have increased by 155 Pg C (90% uncertainty range from 125 to 185 Pg C), leading to ocean acidification (see Chapter 7, Section 7.6.1).

Independent geochemical evidence confirms that the increase in atmospheric CO₂ was primarily driven by fossil-fuel consumption and did not arise from natural sources (see Figure 2.5). Direct measurements starting in the 1990s show a small decrease in atmospheric oxygen (O₂) concentrations, consistent with fossil-fuel burning (as O₂ is consumed during combustion), but inconsistent with a non-oxidative natural source of CO₂, such as the oceans or volcanoes. Second, plants and fossil fuels (derived from ancient plants) have lower ¹³C/¹²C stable isotope ratios than the atmosphere, meaning these sources are relatively depleted in the isotope ¹³C. Burning fossil fuels and plants emits carbon (primarily as CO₂) to the atmosphere with depleted levels of ¹³C. This reduces the ¹³C/¹²C ratio of atmospheric CO₂. Measurements confirm that this is what is occurring. The observed atmospheric CO₂ increase, O₂ decline, and ¹³C/¹²C decrease are larger in the Northern Hemisphere, consistent with the major emissions source of fossil fuels. Together, these lines of evidence produce high confidence that observed atmospheric CO₂ increases are due to human activity (Ciais et al., 2013⁹).

The response of the climate system to increasing greenhouse gases varies from one region to another. As a result, the rates of warming around the world are not the same. These variations are a result of climate processes and feedbacks that depend on local conditions. For example, in Canada, loss of snow and sea ice is reducing the reflectivity (or albedo, see Chapter 2, Box 2.3) of the surface, which is increasing the absorption of solar radiation. This causes larger surface warming than in more southerly regions. Because of this and other mechanisms, Canada is warming faster than the world as a whole — at more than twice the global rate — and the Canadian Arctic is warming even faster — at about three times the global rate.

The last sea ice area in the Arctic during the summer months will be along the northern coasts of Greenland and the Canadian Arctic Archipelago (CAA), as well as areas between the northern islands of the CAA (Figure 5.12), providing an important refuge for sea ice–dependent species. As long as there is sea ice within this region during the summer months, it will continue to be transported southward into the major shipping channels of the CAA, presenting an ongoing potential hazard for shipping in this region, even while the majority of the Arctic is free of sea ice.

The decline of summer Arctic sea ice extent associated with observed warmer temperatures is perhaps the most visible feature of climate change over the past 30 years or more (Comiso, 2012; Fyfe et al., 2013). Arctic sea ice is also thinner because older and thicker MYI has been gradually replaced by younger seasonal ice (Kwok and Cunningham, 2015). Continued declines in both sea ice extent and thickness as a result of further warming from greenhouse gas emissions are projected by the latest state-of-the-art climate models, and this has led to questions about when the Arctic will become free of sea ice during the summer months. The consensus from climate models is that a summertime sea ice–free Arctic could be a reality under a high emissions scenario by mid-century; however, there is considerable regional variability in the timing of projected sea ice–free conditions during the summer months (Laliberté et al., 2016).

The “last ice area” (LIA) refers to regions of the Arctic immediately north of Greenland and the CAA, as well as areas between the northern islands of the CAA (Figure 5.12). The concept of the LIA was borne from climate model simulations that project sea ice within the LIA, even when the rest of the Arctic is virtually sea ice–free during September (Laliberté et al., 2016). Sea ice will persist in the LIA because of the influence of large-scale atmospheric circulation (prevailing winds) on sea ice motion, with the atmospheric Beaufort High driving the counter-clockwise (anti-cyclonic) movement of sea ice in the Beaufort Gyre. As a result, Arctic Ocean sea ice converges against the northern coasts of Greenland and the CAA, creating some the thickest sea ice in the world, with some floes over 5 m thick (Kwok and Cunningham, 2015). This thick sea ice is more resistant to melt under a warming Arctic. So, assuming no major changes in future atmospheric circulation patterns, sea ice will persist in the LIA during the summer, even when the rest of the Arctic is sea ice–free.

When droughts and floods occur, there are usually multiple contributing factors. This makes projecting future changes in these events very challenging. Some contributing factors will be affected by human-induced climate warming, and some will change due to other human influences (such as changes to the landscape). As well, natural climate variability will continue to play a role. As temperatures rise, the threat of drought will increase across many regions of Canada. Projected increases in extreme rainfall in a warmer climate are expected to increase the likelihood of rain-generated flooding in some regions. Snowmelt-related floods are expected to occur earlier in the year, but it is uncertain how projected warming and reductions in snow cover will combine to affect their frequency and magnitude.

As temperatures rise, the threat of drought is projected to increase across many regions of Canada. This includes the southern Canadian prairies and the interior of British Columbia, as well as regions that depend on snowmelt and/or glacial meltwater for their main dry-season water supply. However, there is considerable uncertainty in future drought projections. Similarly, while future warming is expected to affect future factors causing floods, such as extreme precipitation, and the amount and timing of snow/ice melt, it is not straightforward how these changes will interact to affect the frequency and magnitude of future floods across Canada.

Warmer air can hold more moisture. Therefore, in a warmer world, the hydrological cycle is expected to become more intense, with more rainfall concentrated in extreme events and longer dry spells in between (e.g., Houghton, 2004^#). Water availability in Canada is naturally variable, with periodic droughts and floods. Whether both dry and wet extremes will increase in the future in Canada as a result of anthropogenic climate change is a question that challenges climate change adaptation.

Droughts

In a warmer world, most climate models project more frequent, longer-lasting warm spells; overall increased summer dryness in the middle-interior regions of North America; and earlier, less-abundant snowmelt (e.g., Trenberth, 2011^#). Since Canada is projected to warm in all seasons under a range of emission scenarios, drought risk is expected to increase in many regions of the country. In summer, higher temperatures cause increased evaporation, including more loss of moisture through plant leaves (transpiration). This leads to more rapid drying of soils if the effects of higher temperatures are not offset by other changes (such as reduced wind speed or increased humidity) (Sheffield et al., 2012^#). How much summer droughts will increase in frequency and intensity depends on whether future summer precipitation will offset increased evaporation and transpiration. Current climate models suggest that the southern Canadian prairies and the interior of British Columbia will be at a higher risk for drought in the future, but there is considerable uncertainty in future drought projections. Smaller snowpacks and earlier snow and ice melt associated with warming temperatures could increase drought risk in the many snowmelt-fed basins across Canada that rely on this water source, as well as in regions that depend on glacial meltwater for their main dry-season water supply (e.g., Barnett et al., 2005^#). Therefore, as temperatures rise, the threat of drought will increase across many regions of Canada.

Floods

Flooding typically occurs at local to watershed scales. There are several types of floods that affect Canadians, but the most damaging are those related to rivers and those in urban areas (sometimes associated with river flooding). In Canada, the main causes of river floods are intense and/or long-lasting precipitation, snow/ice melt (including rain on snow), river ice jams, or a combination of these causes. Changes to the landscape, such as deforestation (including that caused by fires and tree diseases) and wetland drainage, exacerbate river floods. Urban flooding is usually caused by short-duration intense rainfall events (e.g., those associated with thunderstorms). Urbanization creates large areas of impervious surfaces (roads, parking lots, buildings) that increase immediate runoff, and heavy downpours can exceed the capacity of storm drains (Melillo et al., 2014^#).

While future warming is expected to affect flood-causing factors, it is not straightforward how these changes will interact to affect the frequency and magnitude of future floods across Canada. Projected increases in extreme precipitation (see Chapter 4) are expected to increase the likelihood of rain-generated urban flooding in some regions. Furthermore, when extreme rainfall occurs in drought-stricken areas, the drier and more compact soils are less able to absorb water, thus increasing the likelihood of overland flow and the potential to cause flooding (e.g., Houghton, 2004^#). Projected higher winter and spring temperatures will result in changes to the timing of snow and ice melt and imply a higher potential for rain-on-snow events. The potential for river-ice jams may also increase as a result of winter thaws. However, given that warmer temperatures will be associated with smaller snowpacks, it is unclear how warming will affect the frequency and magnitude of future snowmelt-related floods (e.g., Whitfield, 2012^#). Nonetheless, snowmelt-related floods are expected to occur earlier in the year, on average, in association with higher temperatures. Some evidence for this shift to earlier flood events following snowmelt has already been observed in some Canadian streams over the last few decades (Burn and Whitfield, 2016^#).

Questions for future research

Climate change may also affect weather patterns and storms. For example, climate models predict changes in phenomena that can cause extreme precipitation events, such as atmospheric rivers (narrow bands of concentrated moisture in the atmosphere that enter western Canada from the Pacific Ocean; e.g., Radic et al., 2015^#), and rapidly intensifying storm systems (sometimes referred to as “weather bombs”; e.g., Seiler et al., 2018^#). These changes could influence the future occurrence and location of floods in Canada. In addition, naturally occurring modes of climate variability, including El Niño–Southern Oscillation, the Pacific Decadal Oscillation, and the North Atlantic Oscillation, have been shown to influence Canadian droughts and floods (e.g., Bonsal and Shabbar, 2008^#). Anthropogenic climate change may result in changes to these modes of climate variability over the 21st century, thus affecting future droughts and floods in Canada. All of these topics are active areas of research.