CHAPTER 1

INTRODUCTION

THE PHYSICAL ENVIRONMENTTHAT ORGANISMS INHABIT

The interactions between climate and ecosystemsoccur on different timescales—a day, a year, or longer.On short timescales, we refer to weather, the actual atmosphericsequence of events (storms, wind, daily temperature).Climate is defined as the average of these eventsover years and longer time periods, and is describedby storm frequency and intensity, mean wind, averagetemperature, and so on. Ecological systems experienceand respond to atmospheric events (weather) as well aschange more slowly with average conditions (climate).

A DAY

Consider a single summer day in a forest. As the sun comesup and temperatures warm, trees become active and thechemistry of photosynthesis begins. Some animals begintheir daily activity, while others may seek refuge. Overthe course of a single day, trees transition from removingcarbon from the atmosphere and growing (daytime photosynthesis),to ceasing photosynthesis (nighttime)but releasing some of the day's photosynthetic gain ofcarbon back to the atmosphere during continued metabolism.During photosynthesis, water evaporates from theleaves, cooling not only the leaves but also the air abovethe forest. While trees are growing, changes in their sizeare usually imperceptible over a single day.

A YEAR

Let's expand our perspective to a year. In the spring, leavesbegin to grow and expand, drawing on energy stored theprevious year. The plants begin to take up carbon fromthe atmosphere, but the ecosystem as a whole still mainlyrespires stored carbon back to the atmosphere. As we'llsee later, the atmosphere records this annual cycle ofphotosynthesis and respiration, so cumulatively, theseplant processes affect the entire planet. As the weatherwarms and the season progresses, the carbon balanceshifts, and photosynthesis begins to exceed respiration,leading to net growth of the biosphere. Leaves expand,plants increase in stature, and the daily cycle, describedpreviously, continues within this grander cycle of theseasons. As winter and cold temperatures approach,growth ceases, leaves are shed, and respiration again exceedsphotosynthesis.

A DECADE

Stepping back further, let's look at the ecosystem over adecade. Within the days and years of the decade, we seethe preceding cycles, but we may also see a less orderlypattern. Some years have warmer or wetter conditions,while others are cooler or drier. The vegetation followsthese climate signals, with more growth one year andless another. During a particularly dry year, a fire mayremove most of the plant growth present, leaving baresoil. The carbon the forest stores or loses to the fireaffects the amount of carbon in the atmosphere andeventually affects the climate. We begin to see the firstclues as to how climate change may affect ecosystems, asvariations in the physical resources plants need to grow(water, sunlight, heat) cause variations in the growth ofindividual plants.

A CENTURY

If we observe over many decades, we may note that althoughrainfall varies from one year to the next, the averageamount of rainfall is changing. Although all the treesin the forest grow more in wetter years and less in drieryears, some species are affected more than others. Themore drought tolerant trees grow faster, and they maycome to dominate the forest, inititating a change in itsspecies composition. Thus, the effects of climate on individualforest organisms begin to be translated into alteredrelationships among species.

Imagine a drying trend. The increasingly tallerdrought-tolerant trees begin to shadow the water-lovingspecies and reduce the light available to them for growth.Even in wet years, the drought-tolerant trees now havean advantage, and the entire community of organisms(including the animals that feed on the trees, and thepredators that eat those herbivores) begins to change.

Although climate initiated the change, the interactionsamong the forest's organisms now take hold andcontrol some of the change. Did a water-loving tree dieof drought, or did it perish from inadequate light forgrowth owing to shade from a taller drought-toleranttree? Was this tree death a climate effect or an effect ofplant community processes? Of course, it was both, andin this simplified tale we can begin to see the complexityof climate–ecosystem interactions. As the forest canopygrows and covers more of the landscape, it makes theland surface darker, so it absorbs more sunlight, warmingthe surface more, and actually begins to change thelocal climate. Climate affects the metabolism and behaviorof individual organisms, but these biological changesaffect an organism's interactions with other organisms,and both the physics and the ecology of the system.

THE GLACIAL CYCLE

Climate and ecological change over decades is difficult toperceive, and scientists are just beginning to understandit, but the climate system and life are coupled on longertimescales as well (Barnola et al. 1991). The glacial–interglacialcycles, during which the earth cools andallows the growth of huge ice sheets, and then warms,releasing the water stored in the ice back into the oceans,are familiar. Some of the biological changes that occurover millennia as the earth warms and cools are similarto those described for a century. Some species fail, whileothers prosper, in glacial or warm interglacial climates.On these timescales, changes to marine and terrestrialcarbon cycles have significant effects on climate, actingas controllers and not just responding passively.

However, over millennia, evolutionary change alsooccurs. Within species, cold- or heat-dominant genesmay become more or less common, and entire speciesmay arise or become extinct. Some of these changes mayoccur because of direct effects of climate. For example, aspecies may be unable to adapt to cold conditions, andall members of that species may die or fail to reproduce.Cold may reduce the numbers of a key prey species, leadingto the extinction of a predator, or another predatorspecies may be better able to travel over snow and thusmay drive a competitor species to extinction through itshigher effectiveness in a snowier climate.

THE GEOLOGICAL TIMESCALE

Climate and life also change together on the longest timescales.Paradoxically, these relationships may be themost familiar, as we know, for example, that dinosaursflourished in a warmer past epoch of the planet. On thegeological timescale, species or entire phyla flourish anddecline in synchrony with vast, slow changes in the climate.On these timescales, life affects the geological andgeochemical Earth System, changing rates of erosion (asland plants developed, they anchored the planet's soils);weathering of minerals (by fixing carbon and releasing itin soils as acidic compounds that affect mineral chemistry);and changing the composition of the atmosphere,releasing oxygen, methane, nitrous oxide, and otherchemically or climatically important gases.

THE HUMAN TIMESCALE:THE ANTHROPOCENE

Geological time periods reflect events that are recordedin the rock record, through volcanic or erosional processesand other events that leave global traces evidentto geologists. Recently, scientists have discussed termingthe present the Anthropocene, because the effects ofhuman use of natural resources, construction of citiesand other infrastructure, climate change, and the impactof human-caused mass extinctions on the futurefossil record should be evident to far-future researchers.In trying to understand present climate–ecosystem interactionsthe impacts of humanity are crucial. Human activity can change the way events occur overmany different timescales. Harvesting a forest can instantaneouslyremove most of the wood slowly accumulatedover days to centuries. However, that removalresets the forest's clock and will influence its dynamicsfor—at least—the lifespan of those trees. Human disturbance(forestry, conversion to agriculture) tends tocause rapid change to ecosystems but triggers slow responsesas systems recover biomass and species compositionover decades.

Of course, living systems are responding on all thesetimescales simultaneously. Early efforts to understandclimate and ecosystems took shortcuts and tried to identifydominant influences of one timescale or another, butnow we know that all these processes interact on differenttimescales. Year-to-year differences in crop yield maybe due to just one extreme weather event. Centennialchanges can arise when a fire or drought resets the agestructure of a forest. A long-term trend may drive ecosystemsto a state in which they respond differently to anextreme event. In the western United States, long-termtrends in forest management have changed the sensitivityof forests to drought by allowing thick stands of treesto develop in the absence of fire; when drought comes,the dense forests (which fully use all the water availablein wet years) are more stressed than they would be ifthere were fewer trees. In marine systems, slow changesin climate may influence long-lived fish populations,again changing the vulnerability of the system to rapidchanges in phytoplankton following a climate event suchas El Niño. Examination of the contingent and interactingeffects of events and processes on different timescalesis a major theme of this book and, as we'll see, providesmuch of the interest, challenge, and complexity of thisscience.

This book discusses the role of the earth's living organismsin the Earth System (ESSC 1988), which comprisesthe interacting atmosphere, oceans, lithosphere (soil androcks), cryosphere (snow and ice), and biosphere, allinfluenced by and, most important, ceaselessly interactingwith human activities (see figure 1). The biosphereaffects the other Earth System components and is, inturn, influenced by them in many ways. A few decadesago, most scientists thought that life exists within thegeophysical Earth System but influences it only in minorways. The reality is more complex and more interesting.

Ecosystems and their interactions with climate varygreatly in different physical regions of the planet. Warmand wet climates have abundant growth and great diversityof organisms, and vegetation controls the flux of energyback to the atmosphere. Cold northern regions havesimpler and less diverse systems but store vast amountsof carbon. The cold waters of the north have productivefisheries but lack the complexity and diversity of tropicalreefs. While we observe that climate shapes life on theplanet, great mysteries remain about how life respondsto climate.

The living world, in turn, also shapes the physical andchemical Earth System. The composition of the atmospherereflects the chemistry of life and is far from thechemical equilibrium that would obtain without the oxygenreleased by plant and microbial photosynthesis, thenitrogen converted by microorganisms into the formsthat help warm our planet, and the water mined by treesfrom soils and released back into the atmosphere to coolthe planet's surface.

The interaction of climate and life has been a scientifictopic for centuries and an especially vibrant field ofresearch for the past few decades. However, as the realizationthat our planet's climate is inexorably changinghas dawned on humanity, understanding the effectsof climate on living systems—and how life might affectthe climate changes triggered by fossil fuel burning—hasbecome more than an academic curiosity and is nowneeded to guide adaptation to these changes. Organisms,communities of organisms, and the great planetarybiosphere itself respond to environmental change, andthese changes affect the services the biosphere providesto humanity. While much is known about how the biosphereinteracts with the rest of the Earth System, muchremains unknown, This book focuses on how climate-triggeredbiological changes feed back to the physicaland chemical parts of the Earth System across a widerange of timescales.

The scientific study of Earth is broken up into anumber of disciplines, including atmospheric science,oceanography, ecology, geology, the natural resourcedisciplines of forestry and agronomy, hydrology,and—increasingly—the human studies including anthropology,history, economics, and geography. Often,subdisciplines concentrate on certain timescales as well,with paleoclimatology, paleoecology, and paleooceanographyfocused on the past. The human disciplines historyand archaeology are distinct from studies of thepresent. You may be taking or teaching a course organizedin one of these ways, but understanding climateand ecosystems and studying the Earth System drawson and unifies these approaches. This book is groundedin biology but draws on all these related approaches tostudying our planet. The nexus of these different approachesis an emergent and coherent body of thought,sometimes called Earth System Science, and this book iswritten from that perspective.

CHAPTER 2

THE CLIMATE SYSTEM

CLIMATE, CLIMATE VARIABILITY,AND RESOURCES TO SUPPORTTHE LIVING WORLD

This chapter focuses on the relationship betweenphysical variability in the environment and livingsystems. Organisms need resources from the physicalworld, including heat (to maintain body temperatures),light (to drive photosynthesis and for vision), water (tomaintain hydration and solute balance), and chemicalnutrients (which are linked to climate in complex wayswe'll explore later). Organisms differ in their needs andsensitivity to these physical and chemical resources.

On short timescales, organisms respond to weatherphysiologically and behaviorally. Their growth rate mayvary with temperature and incident sunlight, or their activitymay vary between warmer and colder conditions.On longer climate timescales, animals may adjust theirbehavior and microhabitat selection or change theirranges. Scientists have studied these physiological andbehavioral responses and the fascinating and unique waysorganisms function in the planet's diverse climate zones.

Climate effects can be thought of as a cascade, fromimmediate and direct physical responses that causephysiological or behavioral reactions, to consequent butlonger-term effects caused by changes in interactionsamong organisms in response to the direct effects. Organismshave to respond not only to Earth's diverse climategeography within each of the planet's climate zonesbut also to year-to-year variation and longer trends inclimate within these zones. On these longer timescales(year to year and longer), organisms all respond to climate,but with differing sensitivities.

These differing sensitivities trigger changes in interactionsamong organisms (through competition, herbivory,predation, decomposition, and other processes). Short-term,direct responses produce a cascade of longer-termindirect effects as a result of interactions and feedbacksamong the biota. This idea that immediate direct effectstrigger complex and often surprising long-termconsequences is a central concept of this book, which will beillustrated using the incredibly rich experience of researchersstudying climate and life.

CLIMATE FOR BIOLOGISTS

For a biologist, it is critical to see climate through thesenses and responses of organisms. Conventionally, wethink of climate in terms of temperature, rainfall, sunshine,and wind as measured or forecast in very standardizedways. By contrast, organisms respond to the environmentthrough the availability of resources: energy to fuel photosynthesisand metabolism, water to support hydrationand cooling, and wind-driven motion that may transportmineral resources from great distances, which are all controlledby the processes we term climate. This section is ashort primer on climate from the perspective of an ecologist.It discusses the way in which climate affects organismsand introduces the key variables used to describethose effects. Perhaps even more important, it discussessome of the temporal and spatial patterns of the climatesystem and how these affect living systems, and returns tothe idea of cascades of effects on different timescales.

ENERGY BALANCE

The biological climate, or bioclimate, is largely defined bythe flow of energy (Bonan 2008). Surface temperature isdetermined by the balance of incoming radiation, outgoingradiation, the evaporation of water, and the exchangeof (sensible) heat through conduction and convection.The flux associated with the evaporation of water is oneof the largest fluxes of energy in many ecosystems andis called the latent heat flux. This is also often the largestecosystem flux of water. Latent fluxes occur when heatis used to evaporate water: energy is transferred to thewater as it changes phase from liquid to vapor. Thus, thefluxes of water and energy are tied together in the surfaceenergy balance, which is driven by radiation from thesun, quantified as net radiation. Net shortwave radiationis the solar shortwave radiation absorbed by the surface,after some is reflected based on the albedo, or

S_r = rS_i

where S_r is the net shortwave radiation; r is the albedo, orthe fractional reflectance; and S_i is the incident radiation.The albedo of different Earth materials ranges from verybright (reflecting nearly all sunlight), such as snow (0.8–0.95),to dark, such as water (0.03–0.1). Vegetation typicallyhas a low albedo, reflecting relatively little sunlight(0.05–0.25, depending on vegetation type and season).Longwave (thermal) radiation at the earth's surface, someof which is absorbed and some of which is emitted at alevel proportional to the surface temperature is given by

L_e = σε(T)⁴ + (1 - ε)L_i

where L_e and L_i are the emitted and incident longwaveradiation, respectively; σ is the Stefan-Boltzmann constant(5.6 × 10^-8 W m^-2 K^-4); ε is the emissivity of the landsurface, which is less than 1 (usually between 0.9 and1); and T is Kelvin temperature. The net radiation is theamount of shortwave and longwave radiation absorbedand defines the amount of energy available to drive biologicaland physical processes at the land surface.