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Greening of the Earth (Observations - Global) -- Summary
How have earth's terrestrial plants responded - on average and in their entirety - to the atmospheric temperature and CO2 increases of the past quarter-century? Because of the large number of studies that have addressed this subject, we are treating it on a continent-by-continent basis. In this summary, however, we report the results of studies that have looked at either the world as a whole or groups of more than two continents at the same time.

In one of the earlier studies of the subject that we reviewed on our website, Joos and Bruno (1998) used ice core and direct observations of atmospheric CO2 and 13C to reconstruct the histories of terrestrial and oceanic uptake of anthropogenic carbon over the past two centuries. This project revealed, in their words, that "the biosphere acted on average as a source [of CO2] during the last century and the first decades of this century ... Then, the biosphere turned into a [CO2] sink," which implies a significant increase in global vegetative productivity over the last half of the 20th century.

More recently, Cao et al. (2004) derived net primary production (NPP) values at 8-km and 10-day resolutions for the period 1981-2000 using variables based almost entirely on satellite observations, as described in the Global Production Efficiency Model (GLO-PEM), which consists, in their words, "of linked components that describe the processes of canopy radiation absorption, utilization, autotrophic respiration, and the regulation of these processes by environmental factors (Prince and Goward, 1995; Goetz et al., 2000)." In following this procedure, they learned that over the last two decades of the 20th century, when the heat was on, "there was an increasing trend toward enhanced terrestrial NPP," which they say was "caused mainly by increases in atmospheric carbon dioxide and precipitation."

A year later, Cao et al. (2005) used the CEVSA (Carbon Exchanges in the Vegetation-Soil-Atmosphere system) model (Cao and Woodward, 1998; Cao et al., 2002), forced by observed variations in climate and atmospheric CO2, to quantify changes in NPP, soil heterotrophic respiration (HR) and net ecosystem production (NEP) from 1981 to 1998. As an independent check on the NPP estimate of CEVSA, they also estimated 10-day NPP from 1981-2000 with the GLO-PEM model that uses data almost entirely from remote sensing, including both the normalized difference vegetation index (NDVI) and meteorological variables (Prince and Goward, 1995; Cao et al., 2004). This protocol revealed, in Cao et al.'s words, that "global terrestrial temperature increased by 0.21°C from the 1980s to the 1990s, and this alone increased HR more than NPP and hence reduced global annual NEP." However, they found that "combined changes in temperature and precipitation increased global NEP significantly," and that "increases in atmospheric CO2 produced further increases in NPP and NEP." They also discovered that "the CO2 fertilization effect [was] particularly strong in the tropics, compensating for the negative effect of warming on NPP." Enlarging on this point, they write that "the response of photosynthetic biochemical reactions to increases in atmospheric CO2 is greater in warmer conditions, so the CO2 fertilization effect will increase with warming in cool regions and be high in warm environments." The end result of the application of these models and measurements was their finding that global NEP increased "from 0.25 Pg C yr-1 in the 1980s to 1.36 Pg C yr-1 in the 1990s."

Commenting on their findings, Cao et al. note that "the NEP that was induced by CO2 fertilization and climatic variation accounted for 30% of the total terrestrial carbon sink implied by the atmospheric carbon budget (Schimel et al., 2001), and the fraction changed from 13% in the 1980s to 49% in the 1990s," which indicates the growing importance of the CO2 fertilization effect. Also, they say that "the increase in the terrestrial carbon sink from the 1980s to the 1990s was a continuation of the trend since the middle of the twentieth century, rather than merely a consequence of short-term climate variability," which suggests that as long as the air's CO2 content continues its upward course, so too will its stimulation of the terrestrial biosphere likely continue its upward course.

Using a newly-developed satellite-based vegetation index (Version 3 Pathfinder NDVI) in conjunction with a gridded global climate dataset (global monthly mean temperature and precipitation at 0.5° resolution from New et al., 2000), Xiao and Moody (2005) analyzed trends in global vegetative activity from 1982 to 1998. The greening trends they found exhibited substantial latitudinal and longitudinal variability, with the most intense greening of the globe located in high northern latitudes, portions of the tropics, southeastern North America and eastern China. Temperature was found to correlate strongly with greening trends in Europe, eastern Eurasia and tropical Africa. Precipitation, on the other hand, was not found to be a significant driver of increases in greenness, except for isolated and spatially fragmented regions. Some decreases in greenness were also observed, mainly in the Southern Hemisphere in southern Africa, southern South America and central Australia, which trends were associated with concomitant increases in temperature and decreases in precipitation. There were also large regions of the globe that showed no trend in greenness over the 17-year period, as well as large areas that underwent strong greening that showed no association with trends of either temperature or precipitation. These greening trends, as they concluded, must have been the result of other factors, such as "CO2 fertilization, reforestation, forest regrowth, woody plant proliferation and trends in agricultural practices," about which others will have more to say as we continue.

Working with satellite observations of vegetative activity over the period 1982 to 1999, Nemani et al. (2003) discovered that the productivity of earth's terrestrial vegetation rose significantly over this period. More specifically, they determined that terrestrial net primary production (NPP) increased by 6.17%, or 3.42 PgC, over the 18 years between 1982 and 1999. What is more, they observed net positive responses over all latitude bands studied: 4.2% (47.5-22.5°S), 7.4% (22.5°S-22.5°N), 3.7% (22.5-47.5°N), and 6.6% (47.5-90.0°N).

The eight researchers mention a number of likely contributing factors to these significant NPP increases: nitrogen deposition and forest regrowth in northern mid and high latitudes, wetter rainfall regimes in water-limited regions of Australia, Africa, and the Indian subcontinent, increased solar radiation reception over radiation-limited parts of Western Europe and the equatorial tropics, warming in many parts of the world, and the aerial fertilization effect of rising atmospheric CO2 concentrations everywhere.

With respect to the latter factor, which is featured prominently on our website, Nemani et al. say that "an increase in NPP of only 0.2% per 1-ppm increase in CO2 could explain all of the estimated global NPP increase of 6.17% over 18 years and is within the range of experimental evidence [our italics]." However, they report that terrestrial NPP increased by more than 1% per year in Amazonia alone, noting that "this result cannot be explained solely by CO2 fertilization."

We tend to agree with Nemani et al. on this point, but also note that the aerial fertilization effect of atmospheric CO2 enrichment is most pronounced at higher temperatures (see the four sub-headings under Growth Response to CO2 with Other Variables - Temperature in our Subject Index), rising from next to nothing at a mean temperature of 10°C to a 0.33% NPP increase per 1-ppm increase in CO2 at a mean temperature of 36°C for a mixture of plants comprised predominantly of herbaceous species (Idso and Idso, 1994). For woody plants, we could possibly expect this number to be two (Idso, 1999) or even three (Saxe et al., 1998; Idso and Kimball, 2001; Leavitt et al., 2003) times larger, yielding a 0.7% to 1% NPP increase per 1-ppm increase in atmospheric CO2, which would represent the lion's share of the growth stimulation observed by Nemani et al. in tropical Amazonia.

Be that as it may, the important take-home message of Nemani et al.'s study is that satellite-derived observations indicate that the planet's terrestrial vegetation significantly increased its productivity over the last two decades of the 20th century, in the face of a host of both real and imagined environmental stresses, chief among the latter of which was what climate alarmists routinely claim to be unprecedented CO2-induced global warming, which they routinely represent as being anathema to life on earth. However, the doomsayers are 180 degrees out of phase with reality in this contention, as earth's vegetation has spoken loud and clear - by its ever-increasing growth rate - that it actually loves higher air temperatures and atmospheric CO2 concentrations.

In another approach to the subject, Idso (1995) laid out the evidence for a worldwide increase in the growth rates of earth's forests that has been coeval with the progression of the Industrial Revolution and the rising CO2 content of the atmosphere. The development of this concept began with the study of LaMarche et al. (1984), who analyzed annual growth rings of two species of pine tree growing near the timberline in California, Colorado, Nevada and New Mexico (USA), and who thereby discovered large increases in growth rate between 1859 and 1983, which rates exceeded what might have been expected from climatic trends but were consistent with the global trend of atmospheric CO2. The developmental journey then continued with a study of ring-width measurements of Douglas fir trees in British Columbia, Canada, that also revealed a marked increase in growth in the trees' latter decades (Parker et al., 1987), leading the principal investigator of the project to state that "environmental influences other than increased CO2 have not been found that would explain this [phenomenon]." West (1988) reported much the same thing with respect to long-leaf pines in Georgia, i.e., that their annual growth increments had begun to rise at an unusual rate about 1920, increasing by approximately 30% by the mid-1980s; and he too stated that "the increased growth cannot be explained by trends in precipitation, temperature, or Palmer Drought Severity Index," leaving the rising CO2 content of the atmosphere as the likely cause of the increase in productivity.

Contemporaneously, stands of Scots pines in northern Finland were found to have experienced growth increases ranging from 15 to 43% between 1950 and 1983 (Hari et al., 1984; Hari and Arovaara, 1988). As to the cause of this phenomenon, the researchers stated that "CO2 seems to be the only environmental factor that has been changing systematically during this century in the remote area under study," and it was thus to this factor that they looked for an explanation of their observations.

The next major development in the continuing saga was the finding of Graybill and Idso (1993) that very long ring-width chronologies (some stretching back nearly 1800 years) of high-altitude long-lived bristlecone, foxtail and limber pine trees in Arizona, California, Colorado and Nevada (USA) all developed an unprecedented upward growth trend somewhere in the 1850s that continued as far towards the present as the records extended. In this case, too, like the ones that preceded it, comparisons of the chronologies with temperature and precipitation records ruled out the possibility that either of these climatic variables played a significant role in enhancing the trees' growth rates, strongly implicating the historical rise in the air's CO2 content as the factor responsible for their ever-increasing productivity over the prior century and a half.

Perhaps the most striking evidence of all for the significant 20th-century growth enhancement of earth's forests by the historical increase in the air's CO2 concentration was provided by the study of Phillips and Gentry (1994). Noting that turnover rates of mature tropical forests correlate well with measures of net productivity (Weaver and Murphy, 1990), the two scientists assessed the turnover rates of 40 tropical forests from around the world in order to test the hypothesis that global forest productivity was increasing in situ. In doing so, they found that the turnover rates of these highly productive forests had indeed been rising ever higher since at least 1960, with an apparent pan-tropical acceleration since 1980. In discussing what might be causing this phenomenon, they stated that "the accelerating increase in turnover coincides with an accelerating buildup of CO2," and as Pimm and Sugden (1994) stated in a companion article, it was "the consistency and simultaneity of the changes on several continents that lead Phillips and Gentry to their conclusion that enhanced productivity induced by increased CO2 is the most plausible candidate for the cause of the increased turnover."

Four years later, a group of eleven researchers headed by Phillips (Phillips et al., 1998) reported another impressive finding. Working with data on tree basal area (a surrogate for tropical forest biomass) for the period 1958-1996, which they obtained from several hundred plots of mature tropical trees scattered about the world, they found that average forest biomass for the tropics as a whole had increased substantially. In fact, they calculated that the increase amounted to approximately 40% of the missing terrestrial carbon sink of the entire globe. Hence, they suggested that "intact forests may be helping to buffer the rate of increase in atmospheric CO2, thereby reducing the impacts of global climate change," as Idso (1991a,b) had earlier suggested, and they identified the aerial fertilization effect of the ongoing rise in the air's CO2 content as one of the factors responsible for this phenomenon. Other contemporary studies also supported their findings (Grace et al., 1995; Malhi et al., 1998), verifying the fact that neotropical forests were indeed accumulating ever more carbon; and Phillips et al. (2002) continued to state that this phenomenon was occurring "possibly in response to the increasing atmospheric concentrations of carbon dioxide (Prentice et al., 2001; Malhi and Grace, 2000)."

As time progressed, however, it became less and less popular (i.e., ever more politically incorrect) to report positive biological consequences of the ongoing rise in the air's CO2 concentration; and the conclusions of Phillips and company began to be repeatedly challenged (Sheil, 1995; Sheil and May, 1996; Condit, 1997; Clark, 2002; Clark et al., 2003). In response to those challenges, we published an editorial rebuttal in the 18 Jun 2003 issue of CO2 Science, after which Phillips, joined by 17 other researchers (Lewis et al., 2005b), including one who had earlier criticized his and his colleagues' conclusions, published a new analysis that vindicated Phillips et al.'s earlier thoughts on the subject.

One of the primary concerns of the critics of Phillips et al.'s work was that their meta-analyses included sites with a wide range of tree census intervals (2-38 years), which they claimed could be confounding or "perhaps even driving conclusions from comparative studies," as Lewis et al. (2005b) describe it. However, in Lewis et al.'s detailed study of this potential problem, which they concluded was indeed real, they found that re-analysis of Phillips et al.'s published results "shows that the pan-tropical increase in stem turnover rates over the late 20th century cannot be attributed to combining data with differing census intervals." Or as they state more obtusely in another place, "the conclusion that turnover rates have increased in tropical forests over the late 20th century is robust to the charge that this is an artifact due to the combination of data that vary in census interval (cf. Sheil, 1995)."

Lewis et al. (2005b) additionally noted that "Sheil's (1995) original critique of the evidence for increasing turnover over the late 20th century also suggests that the apparent increase could be explained by a single event, the 1982-83 El Niño Southern Oscillation (ENSO), as many of the recent data spanned this event." However, as they continued, "recent analyses from Amazonia have shown that growth, recruitment and mortality rates have simultaneously increased within the same plots over the 1980s and 1990s, as has net above-ground biomass, both in areas largely unaffected, and in those strongly affected, by ENSO events (Baker et al., 2004; Lewis et al., 2004a; Phillips et al., 2004)."

In a satellite study of the world's tropical forests, Ichii et al. (2005) "simulated and analyzed 1982-1999 Amazonian, African, and Asian carbon fluxes using the Biome-BGC prognostic carbon cycle model driven by National Centers for Environmental Prediction reanalysis daily climate data," after which they "calculated trends in gross primary productivity (GPP) and net primary productivity (NPP)." This work revealed that solar radiation variability was the primary factor responsible for interannual variations in GPP, followed by temperature and precipitation variability, while in terms of GPP trends, Ichii et al. report that "recent changes in atmospheric CO2 and climate promoted terrestrial GPP increases with a significant linear trend in all three tropical regions." In the Amazonian region, the rate of GPP increase was 0.67 PgC year-1 decade-1, while in Africa and Asia it was about 0.3 PgC year-1 decade-1. Likewise, they report that "CO2 fertilization effects strongly increased recent NPP trends in regional totals."

In a review of these several global forest studies, as well as many others (which led to their citing 186 scientific journal articles), Boisvenue and Running (2006) examined reams of "documented evidence of the impacts of climate change trends on forest productivity since the middle of the 20th century." In doing so, they found that "globally, based on both satellite and ground-based data, climatic changes seemed to have a generally positive impact on forest productivity when water was not limiting," which was most of the time, because they report that "less than 7% of forests are in strongly water-limited systems." Hence, and in spite of what climate alarmists routinely describe as unprecedented increases in the "twin evils" of rising atmospheric CO2 concentrations and air temperatures (which some have described as being greater threats to the world than global terrorism or nuclear warfare), there has in fact been what Boisvenue and Running call a significant "greening of the biosphere," and the world's forests in particular.

Last of all, in one final satellite study of the globe, Young and Harris (2005) analyzed, for the majority of earth's land surface, a near 20-year time series (1982-1999) of NDVI data, based on measurements obtained from the Advanced Very High Resolution Radiometer (AVHRR) carried aboard U.S. National Oceanic and Atmospheric Administration satellites. In doing so, they employed two different datasets derived from the sensor: the Pathfinder AVHRR Land (PAL) data set and the Global Inventory Modeling and Mapping Studies (GIMMS) dataset. Based on their analysis of the PAL data, the two researchers determined that "globally more than 30% of land pixels increased in annual average NDVI greater than 4% and more than 16% persistently increased greater than 4%," while "during the same period less than 2% of land pixels declined in NDVI and less than 1% persistently declined." With respect to the GIMMS dataset, they report that "even more areas were found to be persistently increasing (greater than 20%) and persistently decreasing (more than 3%)." All in all, they report that "between 1982 and 1999 the general trend of vegetation change throughout the world has been one of increasing photosynthesis."

As for what has been responsible for the worldwide increase in photosynthesis - which is the ultimate food source for nearly all of the biosphere - the researchers mention global warming (perhaps it's not so bad after all), as well as "associated precipitation change and increases in atmospheric carbon dioxide," citing Myneni et al. (1997) and Ichii et al. (2002). In addition, they say that "many of the areas of decreasing NDVI are the result of human activity," primarily deforestation (Skole and Tucker, 1993; Steininger et al., 2001) and urbanization Seto et al. (2000).

In conclusion, the results of these many studies demonstrate there has been an increase in plant growth rates throughout the world since the inception of the Industrial Revolution, and that this phenomenon has been gradually accelerating over the years, in concert with the historical increases in the air's CO2 content and its temperature.

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Last updated 10 January 2007