Anthropogenic Climate Change The Greenhouse Effect that warms the surface of the Earth occurs because of a few minor constituents of the atmosphere (GHGs) that absorb IR radiation very efficiently. –As a result of human activities, the concentrations of GHGs are increasing. –This will lead to a warmer Earth, the amount depending on forcing and feedback effects.

The Historical Data

Climate Forcing from GHGs Temperature Forcing for uniform increase in GHGs ( o C/ppb)

Radiative Forcing of 1990 Emissions

The 2xCO2 Experiment A standard experiment with global climate models is to calculate the equilibrium climate for the present and for 2xCO2 conditions, and examine the difference between the two states. –Doubled CO2 is used as a surrogate for an equivalent climate forcing from a mix of contributions from many different GHGs.

Equilibrium Modelling In equilibrium studies, the time-dependent nature of CO2 increase and the resulting climate response are ignored. –We only consider the state of the climate when it is in balance with the radiative forcing associated with a particular level of CO2. As long as GHGs continue to increase, climate will never reach equilibrium.

xCO2 Equilibrium The equilibrium climate is much easier to compute, since it is not necessary to: –choose a particular scenario for GHG increases –account for the thermal capacity (lag) of the deep ocean The heat capacity of the atmosphere is equal to less than 3 metres depth of ocean. The lag in the system means that climate change will be out of phase with atmospheric change

Realised vs Committed Change Realised change is that already showing up in instrumental and proxy records of climate, relative to some baseline condition. Committed change is the equilibrium response to an increase in radiative forcing from the increase in GHGs. –Equilibrium simulations cannot be used as actual forecasts of future climate but they can be used to estimate the committed change

Prediction versus Projection Prediction –Forecast, What will be Projection –Scenarios, What could be Climate vs Weather –Climate change is a shift in the average weather that a region experiences

Canada in the 20th Century The climate of Canada became warmer and wetter in the 20th Century –The annual mean temperature increased by about 0.8 o C, with 1998 being the warmest year of the century: almost 2 o C above the normal. –The warming has been greatest in (north) western Canada, and concentrated in the winter and spring seasons.

Night and Day Night-time (minimum) temperatures have warmed more than daytime (maximum) temperatures, reducing the diurnal range. –This is consistent with climate forcing due to an enhanced greenhouse effect.

It’s also Getting Wetter Annual precipitation has increased by 10% across the country –the last 20 years were the wettest of the 20th century –over parts of Arctic Canada, annual precipitation has increased by 25% For more details visit: – fscenarios.htmlhttp:// fscenarios.html

Climate Projections CO2 Scenarios

Scientific Uncertainties Two major sources of uncertainty: –Magnitude (and direction) of feedback effects Lack of empirical data to accurately represent processes in models –Mixing time of oceans (thermal inertia) Realised vs committed change –Other sources of climate change and variation

CO2 Scenarios There are four future emissions pathways defined in IPCC/SRES –A1, A2, B1 and B2 –relating to four different possible future worlds. The four scenarios lead to different atmospheric carbon dioxide concentrations over the next century, but concentrations rise in all cases.

Are they realistic? Assumptions about future greenhouse gas emissions have been selected so as to span about 95 per cent of the range of emissions scenarios published in the current literature.

Range of Scenarios The four emissions scenarios were combined with low, medium and high levels of "climate sensitivity". (The level of warming resulting from a doubling in CO2). –The combinations of low emissions + low climate sensitivity through to high emissions + high climate sensitivity produce a range of future global warming and sea-level rise that span perhaps 90 per cent of likely future climates.

CO2 Scenarios and Global Climate Change The change in annual carbon emissions from energy/industrial sources by 2100 for these four scenarios ranges from a decrease of 4 per cent (B1) - compared to estimated year 2000 emissions - to an increase of about 320 per cent (A2).

How accurate are the projections likely to be? Climate scenarios are not predictions of the future: –they are descriptions of one or more possible future climates. –One important reason for uncertainty is that we do not know how rapidly emissions of global warming gases will grow in the future, –nor do we know exactly how natural climate variability will evolve.

Temperature change: Canada and the Globe

With and Without Aerosols Global Temperature: B-as-U scenarios for GHGs.

Equilibrium results from two GCMs for CO2 doubling

Canada: Temperature Changes

High CO2 Scenarios

Canada warms …. Canada warms substantially more in the future than the global average. This is particularly true of the Arctic region –Annual warming north of 60 o N reaches 0.2 o C/decade even under the B1-low scenario This rate increases to 0.7 o C/decade under the A2- high scenario –Winter warming reaches 1.0 o C/decade for the A2-high scenario –These rates are 50% higher than those south of 60N

Polar Ice Feedback The extent of snow and ice affects the global albedo. Also, the thinning of Arctic Ocean ice releases ocean heat to the atmosphere

Precipitation Changes

The High CO2 Scenarios

Arctic Sea Ice Sea ice extent has been declining since the 1970s and there has been an increase in the length of the summer melt season.

Change in Arctic Ocean: Summer Ice Cover

Historic Seal Level Rise

Rising Seas: The Future One of the most striking consequences of global warming will be the associated rise in global mean sea level.

Global Sea Level Rise The largest contribution to sea-level rise comes from the expansion of warmer ocean water - a process that will continue for centuries to come. Melting of land glaciers account for about 20%. –Complete melting of all land-based glacier ice would ultimately raise sea levels worldwide by about 60 metres!

The last 1000 years Recent reconstruction of NH air temperature based on tree-ring, ice core, coral and historical evidence.

Recent Historical Changes Climate change is not uniform over the Earth Magnitude varies with location

A cooler upper atmosphere Data series are much shorter for upper air temperatures but measurements taken since 1960 suggest the upper atmosphere has cooled by about 0.5 o C/decade. –This pattern is also consistent with an enhanced greenhouse effect.

What does it all mean? The warming clearly observed in the 20th Century has been historically unusual –1998 was probably the warmest year in the last 1000 years –The balance of evidence suggests a discernible human influence on the global climate

Humans and Climate Change Although the precise contribution of human activities to global warming cannot yet be stated with confidence, it is clear that the planet would not be warming as rapidly if humans were not currently emitting about 7 billion tonnes of carbon into the atmosphere each year.