Present global change

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Response of the Earth System to Global Change

The Jmodels elucidate the natural cycling of nutrients and carbon in the oceans and the natural operation of the Earth’s radiation balance. They can also be used to examine some aspects of the operation of the perturbed Earth. Human activities are making large impacts on the ocean, and on the Earth’s radiation balance, and the carbon and radiation budget models in particular given an indication of the likely effects of some of these changes. Some of the changes that can be examined are:

(a) ocean acidification: due to invasion of fossil fuel CO2 into the ocean.

(b) global warming: how increasing the greenhouse effect interacts with other components of the Earth’s radiation balance and heat reservoirs.

(c) future ocean C sink: will any of the multiple ways in which the ocean is being changed make a large difference to how much CO2 it absorbs?

(d) long-term CO2: what legacy will our burning of fossil fuels leave behind for future generations, including our descendants living many thousands of years in the future?

Ocean acidification: this is easy to examine using the carbon model. From the main control panel, select the option to ‘Add Fossil Fuels’. You can then decide whether to add the fossil fuels using a simplified sinewave function or else using historical emissions data and a particular future scenario. These scenarios, generated by the Intergovernmental Panel on Climate Change (IPCC), are derived from particular storylines with different interpretations as to how society will (or will not) curtail CO2 emissions in the future. The A1F1 scenario for instance is a pessimistic scenario assuming that emissions will continue rising steeply, whereas B1 is an optimistic scenario assuming a more ecologically-friendly future world that adopts non-fossil fuel sources. All of the scenarios only specify CO2 emissions between the years 2000 to 2100. A further text box allows you to specify the total overall emissions, which determine the model behaviour beyond the year 2100.

To put the numbers in context, cumulative global human emissions of CO2 to date are about 300 Gt C. If we eventually burn all recoverable fossil fuels then this will increase to an overall total of 4000 or 5000 Gt C. Larger total amounts could eventually ensue if, for instance, methane clathrates on the seafloor release methane into the atmosphere due to global warming.

Once you have specified the amount of fossil fuels, select ‘Run model’ and then examine the graphs of model results that appear on the screen. For more detailed analysis you can choose to ‘Save Simulation’ and then import the saved data file into a package such as Excel.

If you look at page 2 of the graphs you can examine the impacts that your chosen fossil fuel input has on surface and deep ocean pH (top-left panel). You can also see the impact on carbonate ion concentrations (top-right panel) and saturation states (middle-right panel) with respect to the two mineral forms of calcium carbonate (CaCO3): calcite (used by most planktonic calcifiers, such as coccolithophores and foraminifera) and aragonite (used by most coral reefs). There is concern that these CaCO3-using organisms will be detrimentally affected by ocean acidification, especially as saturation states decrease below present-day levels, and even more so when saturation states decrease below one (when seawater becomes undersaturated with respect to calcite and/or aragonite CaCO3). You can get an indication of the consequences of different CO2 emissions scenarios on seawater saturation states by inspecting the graphs after different model runs. If we emit large amounts of CO2, you can see from the model that we will be able to make the surface ocean undersaturated with respect to both mineral forms of CaCO3. We will also be able to make the surface ocean more acidic (lower pH) than the deep ocean for some centuries, a reversal of the normal (natural) situation. However, this model doesn’t tell you about geographical differences (for instance, that surface seawater under-saturation will occur first in polar latitudes).

Global warming: some aspects of global warming can be explored using the radiation budget model, or at least this will become possible once this model is completed.

Future ocean C sink: of the CO2 already emitted to the atmosphere prior to the present day, about half still resides there, but about half has gone elsewhere. Most of the CO2 no longer in the atmosphere has now entered the ocean (causing ocean acidification, see above). A lesser, but still large, amount has fertilised growth of vegetation on land and been taken up into terrestrial biomass and soils. In the future it is suggested that the ocean will continue absorbing CO2 but that the terrestrial biosphere and soils may change from being sinks, as at present, to being sources of CO2. This would exacerbate the rise in atmospheric CO2 and global warming. But how sure are we that the ocean will continue to be an ever larger sponge for anthropogenic CO2? If the ocean were also to turn into a source then this would of course make for even greater trouble.

You can carry out some model tests to examine the sensitivity of the ocean carbon sink to different ways that humans are altering the ocean. As you can see with some experiments, it appears that the ocean is not about to stop being a major sink for CO2. As a start, try doubling the amounts of nutrients (phosphate) flowing down rivers. This is similar to what is actually happening, because fertilisers applied to agricultural land are eventually washed off by rainfall and drain into river which then carry them into the sea.

To carry out this test, first of all run the model with a given amount of fossil fuels. From the main control panel, select the option to ‘Add Fossil Fuels’. Then choose to implement fossil fuels using a simplified sinewave function with a total amount of 4000 Gt C, to be added between years 0 and 400. Click on ‘Apply and Close’ and then open the ‘Set Run Duration and Output Frequency’ panel and set the model to finish at year 200 (we will compare impacts for the end of this century, and assume that CO2 emissions started in 1900). Then click on ‘Run Model’. This completes the default run, in which fossil CO2 has been added to an ocean working as normal. Find out the year 2100 atmospheric CO2 value by clicking on ‘Show Final State’. Write down or remember this number.

The second step is to repeat the run but with ocean functioning disturbed in some way. In the first instance we can double river nutrient input. You can do this if you go to ‘Model Parameters’, select the ‘Rivers’ tab, and then double the value for phosphorus (river nitrogen does not act as a fertiliser in the model, it is only included because it has an impact on alkalinity). Click on ‘Apply and Close’ and then ‘Run Model’. If you select ‘Show Final State’ for a second time and compare the atmospheric CO2 value at the end of this run to the value you wrote down earlier, you should see that it does not differ greatly from the value obtained at the end of the default run. You have demonstrated, assuming the model is sufficiently accurate, that even such a great change as doubling the amounts of nutrients naturally flowing down into the oceans will not make all that much difference to the amount of CO2 in the atmosphere by the end of this century. Remember that the natural level of CO2 in the atmosphere is 280 ppmv, it is currently (year 2008) at about 380 ppmv, and that it is predicted to rise to perhaps 700 or even as much as 1000 ppmv by the end of this century if emissions are not curbed. Changes of just a handful of ppmv are therefore not all that significant in the larger context of these changes.

You can also make other tests of sensitivity of the ocean C sink. A few can be carried out as follows: (a) try doubling both river nitrogen and river phosphorus. (b) Global warming is likely to impede vertical mixing in the ocean because it will make surface waters warmer and therefore more buoyant. What effect will this have? Cancel the previous changes (click on ‘Default’ on the ‘Model Parameters’ panel) and then halve the two mixing rates (select the ‘Physical’ tab and halve the values of KSM and KMD). (c) Ocean acidification could potentially greatly decrease the numbers of CaCO3-using organisms in the ocean. After cancelling the previous change, select the ‘CaCO3’ tab and click on ‘No calcification’.

If you carry out these tests correctly you should find that the rise in atmospheric CO2 repeats inexorably in all runs, regardless (pretty much) of how ocean functioning is impaired or altered. This is the case for changes applied up until year 2100. As ocean changes are applied further into the future the cumulative impacts become greater. But on the whole it seems likely that (1) the ocean will not turn into a carbon source in the foreseeable future (at least while we continue to emit CO2), and also (2)

Long-term CO2: it is commonly assumed that the ‘CO2 problem’ will only persist for a few centuries, and will rapidly dissipate once we stop emitting CO2. This perception is probably false. Model runs with simple box models such as this one suggest otherwise. Try applying 4000 Gt C of fossil fuel CO2 near the beginning of the model run (see above for how to do this) and then set the run duration to a longer interval, for instance 3000 years. If you have time, you could try 30,000 years or even longer. At the end of the run click ‘Show Final State’ and look at the atmospheric CO2 level at the end of the run. Has it returned to its pre-industrial value of 280 ppmv? You can also examine the trajectory of atmospheric CO2 through the model run on the middle-right panel of the first page of the graphs. You should see that the amount of CO2 in the atmosphere has declined following the end of emissions, but that it never goes all the way back down to the starting value. There are valid scientific reasons for suspecting that a fraction of CO2 emissions will remain in the atmosphere for many millennia after emissions cease, possibly even for hundreds of thousands of years. CO2 emissions may leave a legacy that will still be around long after nuclear waste is no longer radioactive. This has implications for issues such as ice-sheet melting, long-term sea-level rise and even for the likelihood of future ice ages.