Our world is changing

Abstract

When I was born in 1951, earth's atmospheric CO₂ concentration was around 310 mg kg⁻¹ (i.e., parts per million), with an annual rate of increase averaging some 0.8 mg kg⁻¹ per year (NOAA, 2024). When I commenced my research career in 1984, atmospheric CO₂ concentration was 340 mg kg⁻¹, with a decadal average increase for the 1980s of 1.6 mg kg⁻¹ per year. In August 2024, atmospheric CO₂ concentration was reported as 423 mg kg⁻¹, with the decadal mean annual increase for the 2010s nearing 2.5 mg kg⁻¹ per year (NOAA, 2024). In the same period, Earth's human population has increased from 2.5 to 8.0 billion. Science says the increase in atmospheric CO₂, together with other trace gases, notably methane and nitrous oxide, will decrease the proportion of insolation received by earth that is reflected back into space, and so warm the planet. The expectation of global temperature increase is the climate change story; it has been told repeatedly in many forums such as the IPCC documents and debated at great length by “believers” and “deniers.” I will not dwell on it here.

There is ample evidence that the predictions are being fulfilled (see, e.g., Figure 2 of Yuan & Hou, 2015). The acceptance of climate change as fact is now mainstream, with the global temperature rise to date frequently stated to be in the vicinity of 1.1°C (IPCC, 2023). Europe is leading the way among nations in transforming lifestyles to achieve carbon neutrality (EU, 2020). The increase in atmospheric CO₂ and population increase are closely linked. Fundamentally, humans need energy to drive their homes, motorcars, and industries; much of this energy comes from burning fossil fuels, thereby releasing CO₂ into the atmosphere that was sequestered in past geological eras. What intuitively perturbs me about the raw NOAA data is that the rate of increase in atmospheric CO₂ concentration is still increasing. After all the international effort, I had thought that the annual rate of global atmospheric CO₂ increase would be falling by now, not still rising.

I turn to the 2023 IPCC 6th Assessment report for guidance as to the status of the collective international effort in climate change mitigation. For me, the report does not join the dots and only increases my feeling of concern. “Summary for policymakers, Figure 5” is telling; it depicts annual global emissions of CO₂ equivalents around 55 Gt per year, and shows that this needs to be halved by 2040 to limit warming to 1.5–2°C. I wondered to myself what the current annual CO₂ increase of 3 mg kg⁻¹ per year would convert into in units of Gt, so I looked up the weight of the earth's atmosphere—5.15 million Gt. Thus, a 3 mg kg⁻¹ annual increase is about 15.5 Gt. Allowing for the additional contribution of methane and nitrous oxide that makes about 25 Gt of CO₂ equivalents, meaning that only about half of the emissions end up in the atmosphere. If the remainder of the CO₂ emissions are being sequestered in sinks like the oceans, causing acidification, that is also of concern. There is much less attention in the literature to what the impacts over time of that process may be, for our planet. It would be good to see in the report a projection of the future outcomes of a “continue as now” scenario. That seems to be missing.

In my home country, New Zealand, a country with a unique economy based on food production from pasture for export, policymakers concerned about CO₂, methane, and nitrous oxide emissions from the agricultural sector have identified tree planting to increase C sequestration and charging farmers for their emissions as key components of a mitigation strategy. This raises a raft of questions. After ramifications through a chain of societal interacting factors, will tree planting in one location with inevitable conversion of pastoral land and shift of that food production elsewhere actually increase global sequestration or will local gains be offset by downstream ramifications? Should food producers be charged for their emissions or should consumers take some responsibility to avoid stimulating production relocation to countries where farmers can produce more cheaply because they are not taxed for their emissions?

The current 1.1°C mean temperature increase seems intuitively innocuous. However, a little reflection brings concern here too. For example, metabolic energy budgeting of three southern North Island beef and lamb farm systems in New Zealand (Gobilik et al., 2021), using data from historic farm records, revealed that pasture harvested by animals fell by about 10% between 1980 and 2010. The change was consistent with the modelled effects on pasture yield based on actual temperature records and one of the farmers whose systems were analysed had already made system configuration changes to reoptimize their farm system considering the changed seasonality of pasture growth. Specifically, there is less grass in summer now than there used to be, to raise lambs to slaughter weight. Ewe hoggets to replace older breeding ewes culled for age are now being reared off farm to compensate. My own calculations suggest that a 1°C temperature rise will increase pasture water use by evapotranspiration by some 20–25 mm over summer, intensifying the summer moisture deficit, which, for the decade 2001–2010, averaged between 74 and 447 mm in different regions of NZ (Matthew et al., 2012). Similarly, another report (Liu et al., 2018) convincingly details warming and associated drying on the Qinghai–Tibet Plateau, with a shift in alpine meadow species composition from sedges towards grasses.

The ramifications of these ongoing changes are unknown. Elsewhere, glaciers are retreating around the globe. Satellite records available from 1979 show that the arctic ice cap has shrunk and thinned massively in the last 45 years (NSIDC, 2024). It may well be gone in summer within the next decade. I don't think we really know what the feedback impacts on earth's oceanic and atmospheric circulation systems of an ice-free arctic ocean might be. As a result of both population pressure on natural resources and the impact of human-induced change, planetary cycling systems are being stretched beyond sustainable operating thresholds. (Rockström et al., 2009). Besides greenhouse gas emissions and associated warming, issues to be addressed in grassland research include ‘green’ water availability (Wang-Erlandsson et al., 2022), soil erosion and salinization, rangeland degradation, biodiversity loss, the formulation and implementation of methodologies to quantify ecosystem services of grasslands and rangelands and more.

Grassland researchers have a disproportionately important role in securing humanity's future as stakeholders and custodians of a planetary resource that will become increasingly important for supply of both income to those working the land and a range of resources and benefits to nearby city dwellers and wider society. As part of our mission “to capture the best thinking that is happening internationally, to create an exchange of ideas among researchers in different countries, and to facilitate excellence in development of new technologies and new solutions,” the Editorial Office of Grassland Research is organising an international forum “Grassland Research: Role in global change and food security,” to be held in Lanzhou on November 18th and 19th, 2024. The programme has 32 speakers comprised of prominent international researchers in key topics, our Editorial Board members and our authors. Please check for the website announcements (https://conferences.koushare.com/grasslandresearch). We are also delighted to advise our authors and readers that we have learned this week that Grassland Research has been accepted for inclusion in the Clarivate Emerging Sources Citation Index and for listing in Web of Science from Issue 1 of Volume 1 (2022).