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Chapter 31.

Implications of

Global Climate Changefor Southern Forests: Can We Separate Fact from Fiction?

Hermann Gucinski, RonNeilson, and Steve McNulty1

Abstract—There is no scientific dispute regardingthe existence of a greenhouse effect. There isno doubt that water vapor, carbon dioxide (CO2),and methane concentrations are greenhousegases. The data showing increases in CO2 in theatmosphere are incontrovertible. Uncertaintiesarise when the Earth’s biological responsesto climate change are to be quantified. Suchuncertainties can be compounded when theresponses of ecosystems, especially forests,are to be delineated. Complex interactionsamong effects of climate change, disturbance,competition, invasive species, managementintervention, land use change, and other actionsmust be clarified by modeling. Model developmenthas improved greatly, but evaluation and validationremain difficult. Model outputs for the Southshow a fairly wide range of potential changesunder scenarios developed from different climatemodels, suggesting that the assumption of steady-state conditions has little likelihood of occurrence.This implies that we should rethink managementapproaches, design research to include newecosystem variables, and seek integratedecosystem knowledge on scales heretoforerarely treated.

INTRODUCTION

There is persistent skepticism about theevidence that greenhouse warming maybe occurring, and this skepticism requires

consideration. This chapter is a semitechnicaldiscussion of the issues, and is designed to shedlight on the question of climate change withoutattempting to present new research information ormake new interpretations of the research findings.

There is no scientific dispute regarding theexistence of a greenhouse effect in planetaryatmospheres such as ours (Raval and Ramanathan1989). Nor is there any doubt that water vapor,carbon dioxide (CO2), and methane are greenhousegases (Ledley and others 1999). Moreover, the datashowing increases in atmospheric concentration ofCO2 are incontrovertible (Keeling and Whorf 1999,Keeling and others 1989). Lastly, it is clear thatthe rise in atmospheric CO2 is largely attributableto the burning of fossil fuels (Andres and others2000, Carbon Dioxide Information Analysis Center2000). Does this prove that there will be climatechange in the form of global warming? It stronglysuggests a change in the Earth’s energy balance,but there is no simple yes or no answer to thequestion about global warming, because thevariables that affect the outcome are many, and theinteractions between Earth system processes havenot been delineated as fully as one might wish.

It is possible to make probabilistic estimatesof likely outcomes, and these have been made(Harvey and others 1990, 1997). The Inter-governmental Panel on Climate Change (IPCC)is thought to be the most authoritative scientificbody that has assembled such estimates. Theacceptance of the IPCC estimates has not beentotal. Uncertainties remain, and skepticismpersists. Because these estimates have vastpolicy implications, the debate has spilled fromthe purely scientific area into the arena of public

1 Assistant Director (now retired), U.S. Department ofAgriculture Forest Service, Southern Research Station,Asheville NC 28802; Research Team Leader, MappedAtmosphere Plant-Soil System Team, U.S. Departmentof Agriculture Forest Service, Pacific Northwest ResearchStation, Corvallis, OR 97331; and Research Ecologistand Project Leader, U.S. Department of AgricultureForest Service, Southern Research Station, Raleigh,NC 27606, respectively.

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debate and governmental policy. The controversyhas led august scientific bodies to make strongstatements regarding the status of climatechange science. In this case a group of NationalScience Academies, representing 16 countries,say the following (excerpted from http://www.royalsoc.ac.uk/files/statfiles/document-138.pdf):

The . . . IPCC represents the consensusof the international scientific communityon climate change science . . . and weendorse its method of achieving thisconsensus. Despite increasing consensuson the science . . . doubts have beenexpressed . . . . We do not consider suchdoubts justified . . . . We support theIPCC’s conclusion that it is at least 90percent certain that temperatures willcontinue to rise, with average globalsurface temperature projected to increaseby between 1.4 and 5.8 oC above 1990levels by 2100 . . . . The balance of thescientific evidence demands effectivesteps now to avert damaging changesto the earth’s climate.

This statement has since been endorsed bythe National Academy of Sciences. Figure 31.1shows the reconstructed temperature record forthe past millennium and the recent temperaturerise coincident with the era of industrialdevelopment.

CLIMATE CHANGE SKEPTICS

Despite the growing scientific consensus,skepticism continues, both in the strictlytechnical areas, where it is part of the

scientific process, and also in the arena of policydevelopment. It is useful to examine the scientificvalidity of the claims made by the skeptics. Theseclaims have appeared in Web sites 2 3 and in abook by Lomborg (1999) and can be summarizedas follows:

• The present uncertainties in Earth processesoverwhelm any confidence in predictions offuture climate. The uncertainties might includemodeling deficiencies, skewing of temperaturerecords due to “heat island” effects, or theswamping of the small increases in CO2 derived

from fossil fuel compared to the large-scaleexchange of CO2 between the atmosphereand the oceans and land

• Factors other than increases in greenhousegases are responsible for warming trends;e.g., Milankovich cycle, changes in the sun’senergy output

• Radiometric data from satellites and high-altitude devices show no warming

Arguments made on scientific grounds aretestable hypotheses, subject to confirmation orrejection. We also see arguments made on policygrounds, or derived from various logical positions,or, at times, logical fallacies. Examples of the latterinclude the use of the fallacy of “condemning theorigin,” e.g., climate change arguments are madeonly by “greens” or radical environmentalists;ergo, the argument must be false, or bydiscrediting the process (“the IPCC is aUnited Nations body, hence subject to politicalconspiracies”). Policy perspectives are not subjectto scientific review, but can be examined in thelight of precedents, and by examining theassumptions upon which they rest. For example,the argument has been made that expendingnational resources on mitigating potential climatechange is unwise because of present uncertainties.One can investigate the underlying assumptionsusing Bayesian formal decision theory. Thus,it may be even more likely that the “no regrets”option, i.e., the avoidance of investments tomitigate climate change that will prove costlyif no climate change occurs, is not the leastexpensive, given the risks. Of course, positionsderived from self-interest, i.e., having a “personal”stake in the outcome, are not subject to scientificdebate, because they bear no relation to thescientific process.

Consider the issue of scientific uncertainties.The IPCC contends that improvements in datacollection and processing, model developments,process-level understanding, and improved large-scale consistency of models with observationsgive a confidence level of better than 90 percentfor the statement that 20th-century globaltemperature increases are the highest of thelast 1,000 years. Moreover, the likelihood thattemperature increases of the 20th century arenot due to climate variability is now estimated atthe 99 percent level (Houghton and others 2001).Such convergence of the analysis demands a betterscientific response than simple rejection of theclimate change hypothesis on the grounds ofuncertainty. Hence it is important to distinguish

2 George C. Marshall Institute. 2002. Homepage.http://www.marshall.org. [Date accessed unknown].

3 Heartland Institute. 2002. http://www.heartland.org.[Date accessed unknown].

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between scientific debate about the implicationsof the remaining uncertainty, and categoricalassertions that uncertainties are large. Moreover,improvements in process-level understanding havelargely overcome the difficulty of calculating smalldifferences between large numbers, and signalscan be observed despite the large flux of CO2between atmosphere, oceans, and the land(Houghton and others 2001).

Factors other than increasing greenhouse gasconcentrations in the atmosphere are affectingthe total radiative energy reaching the Earth’senvelope. Some factors work on cyclical scalesfar longer than any relevant to the postindustrialemissions era, while others, such as 11-year

sunspot cycles, are too short to influence climatechange but do confuse the signal (Houghton andothers 1994, Lean and Rind 1996). Hansen (1998)asserts that the existing record of solar radianceobservation shows variation on the order of 0.1percent, which translates into a forcing of a fewtenths of watts/m2, whereas the forcing fromCO2 increase over preindustrial levels alone isabout 1.4 watts/m2 (Hansen 1998). The relevantobservation is that secular changes are additiveto the human-caused fossil fuel-derived effects,and that the total picture needs to be analyzed.

Local anomalies, such as the heat island effect,have been taken into account in the analysis ofglobal surface temperature data. For example,

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Figure 31.1—The reconstructed Earth surfaceair temperature record for the past millennium(http://www.ipcc.ch/present/graphics/2001syr/ppt/05.16.ppt).

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Hansen and others (2002) point out that observedwarming is greatest at high latitudes, especiallyin Northern Canada, Alaska, and parts of theAntarctic Continent. By comparison, the heat-island effect occurs primarily at middle latitudeswhere the greatest numbers of cities are located.Space-based observations see no stratosphericwarming. This is consistent with thermodynamicprinciples, stratospheric cooling due to ozonedepletion, and effects of sulfate aerosols (NationalAcademy of Sciences 2001) and does not contradictground-based observations (Houghton andothers 2001).

The magnitude of uncertainties concerning theheat island effects and the scale of the correctionsapplied do not invalidate the record of recentrise in global surface temperature, but affectthe absolute magnitude and its standard error.Similarly, upper air temperature measurementsfrom balloons and aircraft do not invalidatepresent assessments. The argument is advancedthat the greenhouse forcing from CO2 increasesis negligible compared to the exchange of thisgas in the seasonal uptake and release from plantphotosynthesis and respiration. This argumentignores both the physics of greenhouse gasesand the principle of superposition. The latterthat says as a first-order approximation the effectsof multiple processes are additive; hence, theadditional atmospheric loading of increasing CO2will be cumulative regardless of the magnitude ofthe annual exchanges.

It is important, therefore, to distinguishskepticism stemming from bad scienceor preconceived conclusion from scientificskepticism that seeks to ensure the knowledgederived is complete, accurate, and inclusiveof all the important variables that are drivingthe system trajectory. Not all uncertainties havebeen eliminated, nor is the potential of unexpectedfuture results negligible. This is particularlytrue when one attempts to map the responses ofterrestrial vegetation to probable external forcing,and even more so when one wants to examineeffects at local to regional scales.

IMPLICATIONS FOR SOUTHERN FORESTS

What, then, are the implications of globalclimate change for the Southern UnitedStates in general, and for its forests

in particular?

The most important inference is that thecontinuation of the present climate is no longerthe only tenable scenario. Instead we are faced

with a range of scenarios that may have differingprobabilities attached to them but cannot bedismissed out of hand. These scenarios ariseprincipally from two processes. One derives fromthe uncertainties regarding future CO2 and othergreenhouse gas emissions, and the other fromdifferences in the climate models themselves,which have been evolving to incorporate a greaternumber of variables and feedback loops overtime. Models have been built that map potentialvegetation based on climate drivers includingtemperature, such as growing degree daysand winter temperature minimums, water(precipitation and soil moisture holding capacity),and photosynthetic radiation. These can use theoutputs of general circulation models (GCM)to drive the vegetation both in equilibrium phase,i.e., after atmospheric CO2 has doubled andequilibrium is attained, and in dynamic phase,mapping the vegetation response to graduallychanging climate (e.g., Bachelet and others2001, VEMAP members 1995). Building in theresponses of the terrestrial ecosystems via theirbiogeochemical cycles is still a developing area(National Academy of Sciences 2001).

As a result, major GCMs exhibit a range ofpossible climate change outcomes, especially at theregional scale, where the remaining uncertaintiesare perhaps greatest. It is instructive to considerthe worst-case scenarios along with the benign oreven desirable outcomes.

A scenario that might pose particular difficulty,because it might convey a false sense of security,is one in which climate change would initiallybe favorable, i.e., one with initial warming andsteady or increasing precipitation, followed bythe development of unfavorable conditions, i.e.,continued warming accompanied by increasingaridity. A hypothesis that may apply for theSouthern United States is the expansion of thesubtropical high-pressure fields, including theBermuda High (Bachelet and others 2001, Dohertyand Mearns 1999). Such expansion would shiftportions of the jet stream northward, which mightunfavorably alter the precipitation and waterregime. Southeastern forests would presumablydecline, and catastrophic fire could well be thechange agent for this decline.

For the Southern United States, ramificationsof such scenarios go beyond forest responses, andinclude changes in water availability among otherhydrologic effects. One may use the global climatemodels to calculate ground-water balance andriver discharge, which together can describe water

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Increasingstress

availability. Figure 31.2 is a depiction of the wateravailability under the scenario of a drier Southonto which population growth rates have beensuperimposed. The figure shows water stressas the ratio of changed availability divided bychanged growth rates. This is only one possibleoutcome of several, selected to show a worst-casesituation. There are benign and even promisingscenarios in which forest expansion is a strongpossibility. Unfortunately, there are no reliabledata at this time on the probability of occurrencefor any of these scenarios (see discussion in Smithand others 2002).

The foregoing implies that our managementstrategies with respect to climate change must bestructured to function in the context of uncertaintyand incomplete information. This is not shockingand not unprecedented; a similar situation existswith respect to other phenomena, such as marketfluctuations or exposure to risk from hurricanesand floods. It simply means that decision theoryapproaches developed elsewhere may be appliedhere and that risk assessment and riskmanagement approaches apply in this arena asthey do in others (Gucinski and McKelvey 1992).

Risk assessment must address two elements,the probability of the occurrence of an event, suchas future drought, and significance (or “value” or“utility” in economics parlance) of the event whenit does occur. For large-scale events with costly or

even catastrophic outcomes, it has been customaryto take a “risk-averse” stance over a “risk-neutral”one, but even that can be problematic in terms ofrequired action. The “zero-infinity” paradox, inwhich the probability of the occurrence of an eventis vanishingly small, but the consequences simplycatastrophic, such as unprecedented wildfire, ora massive earthquake in an urban area, requirespreparatory action regardless of likelihood.

What are the implications of potential climatechange for risk management?

The study of disturbance processes, and theirlessening or accentuation by changing climate,is still relatively young. Wildfire frequency andintensity is being modeled as a response to theeffects of climate change on forest ecosystems(Lenihan and others 1998). McNulty (2002)has described effects of hurricane on southernforests. Management strategies for resistanceto hurricanes include planting (or encouragedregeneration) of deeper rooted and salt-tolerantspecies, and denser forest stocking (or lowerthinning levels). On the other hand, managementstrategies for southern forest response to fireinclude removal of understory, planting (orencouraged regeneration) of species that aremore fire resistant, and lower stocking—or higherthinning levels. It appears that improved forestmanagement and better policy is also neededto improve postfire and posthurricane salvage.

Figure 31.2—Predicted change in watersupply stress defined as the ratio of the2025 demand and supply divided by the1990 ratio of demand and supply. Higherstress is the result of both increasingdemand (population pressure) and thechanged supply (climate driven) (McNultyand others 2004).

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However, increased stocking needed for improvedhurricane resistance increases fuel loads andis detrimental to fire suppression.

Potential effects of climate change onopportunistic species, especially invasives,are being studied, as are fragmentation-relatedbarriers to plant migration, limits on seed,disruption of pollinators, and other potentialproblems. The better we understand the manyfacets of the possible responses to potentialclimate change, the better our position toweigh courses of action that remain open.

We believe that the following may serve asuseful starting points for further exploration ofpossible options for mitigating negative climatechange impacts on Southern U.S. forests:

• Manage forests for low-probability climatescenarios that have large-scale consequences.In this case, diversity may bring resiliency, andultimately sustainability. This strategy may beespecially appropriate for the Southern UnitedStates, where the rotation period for reachingharvest potential is relatively short

• Weigh the risks of omission against thoseof commission. Sometimes the risksincurred through inaction are greater thanby implementing active approaches early,especially when these approaches wouldbe beneficial regardless of the effects ofimpending climate change

• Analyze the potential cost of delaying actionin the hope of obtaining better informationwhen the delay may eliminate viable options.This is the argument advanced by the NationalAcademies in endorsing the IPCC findings.Delaying action until there is greater certaintyabout the potential effects of climate changemay have its own costs. Of course, this doesmean that additional information should notbe sought or consulted

• Be aware that risk assessment is influencedby both objective and subjective elements, andthat consistency in assessment approaches willimprove our chances of meeting our objectives

CONCLUSIONS

Approaches in which potential global climatechange is treated as a set of future risks haveoften been ignored, and certainly have not

been used adequately to assess possible impactsin the Southern United States. The existence ofdecision-theory frameworks, and their use in othersectors, makes this a viable option for managers,

who can also benefit from additional researchin the science community. If by improving ourunderstanding of the risk spectrum now andapplying the insights gained in the planningprocess, we may have many more optionsin the near term. Future climate changeconstraints may limit the choices for climatechange mitigation considerably.

ACKNOWLEDGMENTS

The authors are grateful for the assistanceprovided by Ms. Jennifer Moore, Dr. GeSun, and Dr. John Bartlett with the analysis

and creation of figure 31.2. We also appreciate thecomments and suggestion of the two anonymousreviewers for this chapter.

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