J. G. Shepherd

Exploiting natural populations in an uncertain world

Abstract We consider some aspects of the way random environmental variability can affect fish and other natural populations that are being harvested for sustained yield. A spectrum of 8 stock-recruitment curves, culled from the fisheries literature, is used to study the probability distribution of yield Y as a function of the level of exploitation or “effort” E. If the environmental noise enters via the intrinsic growth rates (“density-independent noise”), the 8 curves are qualitatively in accord in predicting that absolute levels of fluctuation in Y increase as E increases; these trends become strongly marked once exploitation is significantly in excess of the maximum sustained yield (MSY) level. The quantitative details, however, depend on the specifics of the stock-recruitment curve (the CV or relative fluctuation of Y can increase or decrease as E increases toward the neighborhood of the MSY level, and the CV can increase relatively dramatically or relatively gradually as E increases beyond this level). The effects of density dependent noise, and of time delays in population regulatory processes, are also briefly discussed. Broadly, the analysis suggests that environmental unpredictability can pose “portfolio theory” trade-offs between desired levels of average yield and of fluctuations or risk. What seems really needed is not further mathematical refinement, but rather robustly self-correcting strategies that can operate with only fuzzy knowledge about stock levels and recruitment curves.

Ocean fertilization: a potential means of geoengineering?

The oceans sequester carbon from the atmosphere partly as a result of biological productivity. Over much of the ocean surface, this productivity is limited by essential nutrients and we discuss whether it is likely that sequestration can be enhanced by supplying limiting nutrients. Various methods of supply have been suggested and we discuss the efficacy of each and the potential side effects that may develop as a result. Our conclusion is that these methods have the potential to enhance sequestration but that the current level of knowledge from the observations and modelling carried out to date does not provide a sound foundation on which to make clear predictions or recommendations. For ocean fertilization to become a viable option to sequester CO2, we need more extensive and targeted fieldwork and better mathematical models of ocean biogeochemical processes. Models are needed both to interpret field observations and to make reliable predictions about the side effects of large-scale fertilization. They would also be an essential tool with which to verify that sequestration has effectively taken place. There is considerable urgency to address climate change mitigation and this demands that new fieldwork plans are developed rapidly. In contrast to previous experiments, these must focus on the specific objective which is to assess the possibilities of CO2 sequestration through fertilization.

Measurements of the direct deposition of sulphur dioxide onto grass and water by the profile method

Measurements have been made of the rate of direct deposition of sulphur dioxide onto grass and water by the profile method. Over grass the deposition velocity is limited by a surface resistance which varies between 80 and 300 s m−1 and is comparable with stomatal resistance. Over water the deposition velocity is roughly proportional to wind speed, as would be expected if it were limited only by atmospheric resistance. It is however somewhat smaller than theoretical estimates for this situation, being about 11000 of the wind speed at a height of a few metres.

The sensitivity of age-structured populations to environmental variability

Abstract The sensitivity of a periodically reproducing population with overlapping generations to fluctuations in its environment is examined with a model that preserves the essential features of the age structure of the population. The effect of fluctuations is introduced by perturbing the recruitment of young individuals. This is appropriate for many populations of interest. It is found that the return time is in very poor correspondence with the populations sensitivity to noise and, except for determining whether a system is either stable or unstable, is of little use. The results are affected very little by the preservation or not of age structure within the adult population, and only in detail by the use of discrete rather than continuous-time models. The determining factors are the form of the stock-recruitment relationship (particularly the degree of density dependence) and the way in which noise is introduced. For a specific example based on the North Sea herring, we find that sensitivity to noise increases monotonically as exploitation increases, and this is primarily determined by the behavior of the dominant eigenvalue rather than the subdominant modes of the response. Such an increase in sensitivity is undesirable in managed populations. The variance of the population and yield depend upon the density-dependent nature of the noise, and the form of this is particularly important for overexploited stocks. In general there can be a tradeoff between yield and stability of yield.

Sea Surface and High-Latitude Temperature Sensitivity to Radiative Forcing of Climate over Several Glacial Cycles

AbstractA compilation is presented of global sea surface temperature (SST) records that span around one glacial cycle or more, and it is compared with changes in the earth’s radiative balance over the last 520 000 years, as determined from greenhouse gas concentrations, albedo changes related to ice sheet area and atmospheric dust fluctuations, and insolation changes. A first scenario uses global mean values for the radiative changes, and a second scenario uses zonal means for 10° latitude bands for a more regionally specific perspective. On the orbital time scales studied here, a smooth increase of SST response from the equator to high latitudes is found when comparison is made to global mean radiative forcing, but a sharply “stepped” increase at 20°–30° latitude when comparing with the more regionally specific forcings. The mean global SST sensitivities to radiative change are within similar limits for both scenarios, around 0.8 ± 0.4°C (W m−2)−1. Combined with previous estimates of 1.3–1.5 times strong...

Ecosystem Impacts of Geoengineering: A Review for Developing a Science Plan

Geoengineering methods are intended to reduce climate change, which is already having demonstrable effects on ecosystem structure and functioning in some regions. Two types of geoengineering activities that have been proposed are: carbon dioxide (CO2) removal (CDR), which removes CO2 from the atmosphere, and solar radiation management (SRM, or sunlight reflection methods), which reflects a small percentage of sunlight back into space to offset warming from greenhouse gases (GHGs). Current research suggests that SRM or CDR might diminish the impacts of climate change on ecosystems by reducing changes in temperature and precipitation. However, sudden cessation of SRM would exacerbate the climate effects on ecosystems, and some CDR might interfere with oceanic and terrestrial ecosystem processes. The many risks and uncertainties associated with these new kinds of purposeful perturbations to the Earth system are not well understood and require cautious and comprehensive research.

The long-term legacy of fossil fuels

Fossil fuels will have large impacts on ocean chemistry and climate during the period while they are being burnt (and carbon dioxide emitted) in large amounts. It is frequently assumed that these impacts will fade away soon thereafter. Recent model results, by contrast, suggest that significant impacts will persist for hundreds of thousands of years after emissions cease. We present a new analysis that supports these model findings by elucidating the cause of this ‘fossil fuel hangover’ phenomenon. We explain why the carbonate compensation feedback is atypical, compared to other feedbacks, in the sense that convergence is back towards a new steady-state that is distinct from the starting state. We also calculate in greater detail the predicted implications for the future ocean and atmosphere. The post-fossil fuel longterm equilibrium state could differ from the pre-anthropogenic state by as much as 50% for total dissolved inorganic carbon and alkalinity and 100% for atmospheric pCO2, depending on the total amount of future emissions.

Fishing effort control: could it work under the common fisheries policy?

The system of managing fisheries under the CFP using TACs and quotas has not been sufficiently effective, and is no longer adequate. Direct control of fishing effort has always been a possible alternative, but has not been implemented except in special cases because of the difficulties of measuring and comparing the fishing effort of different vessels and fishing gears, and ensuring fair sharing of the resources available. I suggest that both of these problems can be overcome by adopting a scheme based on uniform pro rata adjustments to individual vessel entitlements to fishing effort, based on track records, and thus also maintaining the important principle of relative stability. Such a scheme is well adapted to a system of regional management of the fisheries, could be effectively enforced using satellite monitoring, and should be seriously considered as an alternative to TACs and quotas in the CFP after 2002.

Low efficiency of nutrient translocation for enhancing oceanic uptake of carbon dioxide

Anthropogenic emissions of carbon dioxide (CO2) are steadily increasing the concentration of this greenhouse gas in the Earths atmosphere. The possible long-term consequences of this elevated concentration have led to proposals for a number of large-scale geoengineering schemes that aim to enhance or augment natural sinks for CO2. One such scheme proposes deploying a large number of floating “pipes” in the ocean that act to translocate nutrient-rich seawater from below the mixed layer to the oceans surface: the nutrient supplied should enhance the growth of phytoplankton and consequently the export of organic carbon to the deep ocean via the biological pump. Here we examine the practical consequences of this scheme in a global ocean general circulation model that includes a nitrogen-based ecosystem and the biogeochemical cycle of carbon. While primary production is generally enhanced by the modeled pipes, as expected, the effect on the uptake of CO2 from the atmosphere is much smaller, may be negative, and shows considerable spatiotemporal variability.

Geoengineering the climate: an overview and update

The climate change that we are experiencing now is caused by an increase in greenhouse gases due to human activities, including burning fossil fuels, agriculture and deforestation. There is now widespread belief that a global warming of greater than 2°C above pre-industrial levels would be dangerous and should therefore be avoided. However, despite growing concerns over climate change and numerous international attempts to agree on reductions of global CO2 emissions, these have continued to climb. This has led some commentators to suggest more radical ‘geoengineering’ alternatives to conventional mitigation by reductions in CO2 emissions. Geoengineering is deliberate intervention in the climate system to counteract man-made global warming. There are two main classes of geoengineering: direct carbon dioxide removal and solar radiation management that aims to cool the planet by reflecting more sunlight back to space. The findings of the review of geoengineering carried out by the UK Royal Society in 2009 are summarized here, including the climate effects, costs, risks and research and governance needs for various approaches. The possible role of geoengineering in a portfolio of responses to climate change is discussed, and various recent initiatives to establish good governance of research activity are reviewed. Key findings include the following. — Geoengineering is not a magic bullet and not an alternative to emissions reductions. — Cutting global greenhouse gas emissions must remain our highest priority. (i) But this is proving to be difficult, and geoengineering may be useful to support it. — Geoengineering is very likely to be technically possible. (i) However, there are major uncertainties and potential risks concerning effectiveness, costs and social and environmental impacts. — Much more research is needed, as well as public engagement and a system of regulation (for both deployment and for possible large-scale field tests). — The acceptability of geoengineering will be determined as much by social, legal and political issues as by scientific and technical factors. Some methods of both types would involve release of materials to the environment, either to the atmosphere or to the oceans, in areas beyond national jurisdiction. The intended impacts on climate would in any case affect many or all countries, possibly to a variable extent. There are therefore inherent international implications for deployment of such geoengineering methods (and possibly also for some forms of research), which need early and collaborative consideration, before any deployment or large-scale experiments could be undertaken responsibly.