Using data from direct observations, experimental mesocosms, field experiments, and complex computer models, the IPCC has made a very strong case supporting the hypothesis that human behavior is leading to rapid and substantial climate change. One important anthropogenic effect is changes to the carbon cycle, primarily greater CO2 export into the atmosphere from industrial activity. In recent years, both oceans and terrestrial sources have taken up some of this excess CO2, but ocean uptake is particularly problematic, because it leads to acidification. There are many other important greenhouse gases that influence Earth’s surface temperatures, including methane, nitrous oxide, ozone, and a diverse group of halocarbons. Though less abundant, these gases have a much greater global warming potential than CO2, on a per molecule basis. Many effects of greenhouse gases on global climate are complex; for example, a particular halocarbon can increase and decrease surface temperatures via different mechanisms. There are many different types of climate models that use the movement of the atmosphere around Earth, and the interaction of the atmosphere with the oceans and with biological processes, to project future climate. Though there are quantitative differences between the projections of each model, these models all project a much warmer and wetter global climate over the next century, with northern latitudes experiencing the greatest impact of climate change.

This book was written primarily because of concerns that our climate is changing, and many of those concerns centre on changes in the composition of our atmosphere, especially the rapid increase in the carbon dioxide concentration. Methane is also a concern. In this chapter we will examine the major exchange processes that cycle key elements through our environment, from the atmosphere and oceans, to the biosphere, and even the ‘solid’ Earth, and back again. Our primary focus will, of course, be on the key element carbon, as this encompasses both carbon dioxide and methane. We will also look at the fascinating element nitrogen, and the greenhouse gas nitrous oxide. While our primary focus will be on the natural biogeochemistry of these gases, we will present some of the latest data on how these gases have been increasing in our atmosphere in recent decades.

Abstract: During the past two decades, there have been significant efforts to better quantify emissions of environmentally important trace gases along with their trends. In particular, there has been a clear need for robust estimates of emissions on policy-relevant scales of trace gases that impact air quality and climate. This need has driven the expansion of the observing network to better monitor the changing composition of the atmosphere. This chapter will discuss the use of various data assimilation and inverse modelling approaches to quantify these emissions, with a focus on the use of satellite observations. It will discuss the inverse problem of retrieving the atmospheric trace gas information from the satellite measurements, and the subsequent use of these satellite data for quantifying sources and sinks of the trace gases.

The processes by which forests influence large-scale climate are well known, but the specific response to changes in forest cover varies with background climate (e.g., tropical, temperate, boreal), the extent of forest change and type of conversion, and time of year, and differs between day and night. Forests remove carbon dioxide from the atmosphere, thereby lessening planetary warming. Forests also affect climate through exchanges of energy, water, and momentum with the atmosphere, which can warm or cool the climate depending on geographic location, time of year, and time of day. There is a distinct latitudinal pattern from tropical to temperate to boreal forests, with different influences on temperature and different underlying mechanisms. In general, forests cool the daytime surface climate during the growing season through evapotranspiration and other non-radiative processes, and they warm the nighttime climate. Outside of the growing season, forests are generally warmer than open fields, especially in locations where snow is present. Further climate influences occur through chemical emissions that produce aerosols. Much of this understanding comes from models of Earth’s climate.

The climate benefit of forests is mostly recognized in their removal of carbon dioxide from the atmosphere. Over the course of a tree’s lifetime, the accumulated carbon in biomass is carbon removed from the atmosphere. Species differ in growth rate, size at maturity, and longevity, but the basic principle of biomass accumulation over the lifetime of a tree is the basis for using forests to remove carbon dioxide from the atmosphere. Decomposition of organic material in the soil emits carbon dioxide and reduces the net carbon gain by forests. Wildfires, insect outbreaks, logging, and other disturbances also release carbon dioxide to the atmosphere. The combination of these processes – carbon gain from biomass growth; carbon loss from the soil and from disturbances – makes some forests a sink for atmospheric carbon; the forests have a net gain of carbon annually. Other forests are a source of carbon, in which there is a net loss of carbon to the atmosphere. Forests are, at a global scale, an annual carbon sink, which reduces the accumulation of carbon dioxide in the atmosphere. Nature-based solutions to mitigate climate change aim to enhance the carbon sink.

The Earth surface carbon cycle consists of regular flows between the major sinks: atmosphere, ocean, soils and biota. It is in balance, except for flows to and from long-term storage (methane formations, deep ocean, lithosphere), which comprise the deep Earth carbon cycle. At the beginning of life in earth history, about 500 million years ago, the atmosphere was carbon-rich, and Earth temperatures were vastly hotter than today. Over time most of this carbon was withdrawn from the surface carbon cycle through ocean sedimentation, reactions with weathering rock and migration of organic matter to the lithosphere, where they were transformed into fossil fuels. This eventually produced the temperate Earth, which facilitated human evolution, but now, by extracting and burning fossil energy, humans are recreating conditions for the hotter Earth of earlier times. These direct carbon emissions can be amplified by feedback mechanisms, whereby initial warming releases carbon stored in formerly stable methane deposits and alters the carbon cycle so as to increase the size of the atmospheric sink relative to other sinks or changes in the Earth’s albedo. The politics of controlling emissions is further complicated by lags in warming and subsequent climate impacts.

This is the first of two chapters that focus on mitigation or emissions reduction. Chapter 6 addresses the primary cause of climate change, which is the emission of CO2 that derives from the combustion of fossil fuels. We review the oppositional forces of exploration and consumption in respect of fossil fuel reserves and the concepts of peak oil, gas, and coal. We turn next to the carbon cycle and how it is altered by widespread fossil fuel combustion. The low hanging fruit in mitigation is to electrify everything within reason and to simultaneously decarbonize our electrical supply. However, grid limitations, scant electrical storage capacity, and the intermittent nature of wind and solar combine to make deep decarbonization a more difficult challenge than is often appreciated. These factors along with the distinct consideration of national energy security help explain why fossil fuels are likely to persist late into this century as a source of dispatchable electric power. Finally, there are economic sectors such as aviation and steel making that lend themselves poorly to electrification and will likely remain recalcitrant sectors.

The movement of gas species across the air–water interface is a central aspect of biogeochemical cycling and plays a critical role in controlling not only the composition of the atmosphere, but the chemistry of aquatic and marine systems. This chapter shows how kinetic rate laws describing the transfer of gas species into and out of aqueous solution can be integrated into multicomponent chemical reaction models and shows a fully worked calculation, using carbon dioxide efflux from a biologically active lake as an example.

Human activities in the land sector produce the second highest level of greenhouse gas emissions globally and the highest in some countries, but the land is also a major sink for anthropogenic emissions. This land carbon sink is at risk because plant growth, survival and distributions are dependent on climate conditions and particularly extreme events: higher temperatures, reduction in water availability in some areas and flooding in others, droughts and wildfires. Agricultural and forest productivity are affected by the combined pressures of climate change, demand for land resources for food production, deforestation and degradation of natural systems, and population pressure. The land sector provides numerous opportunities for mitigation, many of which are cost-effective and can be implemented quickly. However, the land sink has limits because the uptake of carbon represents replacing the stock that was previously depleted by human activities, and productivity is limited by nutrient and water availability. Potential mitigation activities should be assessed in terms of the magnitude, timeframe and cost of the changes in carbon stocks.

Climate modeling developed further at the National Center for Atmospheric Research (NCAR) in Boulder, Colorado. The laws of physics that form the foundation of weather and climate models imply strict conservation of properties like mass, momentum, and energy. A household budget analogy can be used to explain these conservation requirements, which are stricter for climate models as opposed to weather models. A mismatch in the energy transfer between atmospheric and oceanic models that were part of a climate model led to a correction technique developed in the 1980s known as flux adjustment, which violated energy conservation. Subsequent improvements in climate models obviated the need for these artificial flux adjustments. Now we have more complex models, known as Earth System Models, that include biological and chemical processes such as the carbon cycle. The concept of the constraining power of models is introduced.

The first chapter introduces a new geological period – the Anthropocene – and explains its origins its timing and importance in emphasizing the current biotic crisis. The current idea that we are in the sixth extinction is criticized because (1) the current level of extinction isn’t high enough to justify it and (2) it limits today’s biotic crisis to just those species that have gone extinct and does not include those under threat. The term defaunation is used in preference because it provides a clearer picture of the situation we find ourselves in. It does this by including both species that have gone extinct and those under threat of extinction. The important concept of the Earth System is outlined and its role in maintaining planetary conditions explained. The link between sudden shifts in Earth System due to climatic forcings and mass extinctions is explained.

Fundamental knowledge about the processes that control the functioning of the biophysical workings of ecosystems has expanded exponentially since the late 1960s. Scientists, then, had only primitive knowledge about C, N, P, S, and H2O cycles; plant, animal, and soil microbialinteractions and dynamics; and land, atmosphere, and water interactions. With the advent of systems ecology paradigm (SEP) and the explosion of technologies supporting field and laboratory research, scientists throughout the world were able to assemble the knowledge base known today as ecosystem science. This chapter describes, through the eyes of scientists associated with the Natural Resource Ecology Laboratory (NREL) at Colorado State University (CSU), the evolution of the SEP in discovering how biophysical systems at small scales (ecological sites, landscapes) function as systems. The NREL and CSU are epicenters of the development of ecosystem science. Later, that knowledge, including humans as components of ecosystems, has been applied to small regions, regions, and the globe. Many research results that have formed the foundation for ecosystem science and management of natural resources, terrestrial environments, and its waters are described in this chapter. Throughout are direct and implicit references to the vital collaborations with the global network of ecosystem scientists.