Evaluation of Wetland CH₄ in the JULES Land Surface Model Using Satellite Observations

Wetlands are the largest natural source of methane. The ability to model the emissions of methane from natural wetlands accurately is critical to our understanding of the global methane budget and how it may change under future climate scenarios. The simulation of wetland methane emissions involves a complicated system of meteorological drivers coupled to hydrological and biogeochemical processes. The Joint UK Land Environment Simulator (JULES) is a process-based land surface model that underpins the UK Earth System Model and is capable of generating estimates of wetland methane emissions. In this study we use GOSAT satellite observations of atmospheric methane along with the TOMCAT global 3-D chemistry transport model to evaluate the performance of JULES in reproducing the seasonal cycle of methane over a wide range of tropical wetlands. By using an ensemble of JULES simulations with differing input data and process configurations, we investigate the relative importance of the meteorological driving data, the vegetation, the temperature dependency of wetland methane production and the wetland extent. We find that JULES typically performs well in replicating the observed methane seasonal cycle. We calculate correlation coefficients to the observed seasonal cycle of between 0.58 to 0.88 for most regions, however the seasonal cycle amplitude is typically underestimated (by between 1.8 ppb and 19.5 ppb). This level of performance is comparable to that typically provided by state-of-the-art data-driven wetland CH4 emission inventories. The meteorological driving data is found to be the most significant factor in determining the ensemble performance, with temperature dependency and vegetation having moderate effects. We find that neither wetland extent configuration out-performs the other but this does lead to poor performance in some regions. We focus in detail on three African wetland regions (Sudd, Southern Africa and Congo) where we find the performance of JULES to be poor and explore the reasons for this in detail. We find that neither wetland extent configuration used is sufficient in representing the wetland distribution in these regions (underestimating the wetland seasonal cycle amplitude by 11.1 ppb, 19.5 ppb and 10.1 ppb respectively, with correlation coefficients of 0.23, 0.01 and 0.31). We employ the CaMa-Flood model to explicitly represent river and floodplain water dynamics and find these JULES-CaMa-Flood simulations are capable of providing wetland extent more consistent with observations in this regions, highlighting this as an important area for future model development.

δ 13 C methane source signatures from tropical wetland and rice field emissions

The atmospheric methane (CH 4 ) burden is rising sharply, but the causes are still not well understood. One factor of uncertainty is the importance of tropical CH 4 emissions into the global mix. Isotopic signatures of major sources remain poorly constrained, despite their usefulness in constraining the global methane budget. Here, a collection of new δ 13 C CH 4 signatures is presented for a range of tropical wetlands and rice fields determined from air samples collected during campaigns from 2016 to 2020. Long-term monitoring of δ 13 C CH 4 in ambient air has been conducted at the Chacaltaya observatory, Bolivia and Southern Botswana. Both long-term records are dominated by biogenic CH 4 sources, with isotopic signatures expected from wetland sources. From the longer-term Bolivian record, a seasonal isotopic shift is observed corresponding to wetland extent suggesting that there is input of relatively isotopically light CH 4 to the atmosphere during periods of reduced wetland extent. This new data expands the geographical extent and range of measurements of tropical wetland and rice δ 13 C CH 4 sources and hints at significant seasonal variation in tropical wetland δ 13 C CH 4 signatures which may be important to capture in future global and regional models. This article is part of a discussion meeting issue ‘Rising methane: is warming feeding warming? (part 2)’.

The Hydrological Viability of Te Harakiki Wetland, Waikanae

<p>Wetlands are unique natural resources that play an important role in the hydrological cycle. There is a dynamic link between wetland hydrology and inputs from both surface and groundwater resources. Shallow groundwater abstraction near the Te Harakiki wetland at Waikanae has the potential to impact on the wetland' hydrosystem. To assess the likelihood of this occurring, a detailed analysis of recent changes, the hydrological regime, and the water balance of the Te Harakiki Wetland system was undertaken. The hydrological regime of the wetland system was assessed by various monitoring sites established around Te Harakiki to measure rainfall, soil moisture, surface and groundwater levels. Analysis of (decadal) historical aerial photographs allowed changes in spatial extent of the open water habitat (lagoon) and the urban area of Waikanae Beach. Comparisons were made between wetland extent, population increase and urban area expansion. These data, together with a simple water balance, and historical climatic records, were used to explain the drastic decrease in wetland extent. Climatic factors and goundwater are the major driving forces behind the wetland's hydrologic regime. The surface water outflow from the system is greater than the surface water inflow, but this may be affected by the tides. The surface and groundwater systems in the area are closely linked. They have similar responses to rainfall events. Groundwater abstraction in the area appears to have minimal impact on the water level within the wetland. The exact nature and extent of abstraction around the wetland is unknown. The reduction in flood pulsing as a result of channel modification, and the fragmentation of the area for the construction of the oxidation ponds are the likely explanation. The current restoration efforts in regard to controlling pest species and excluding stock from the wetland have halted the decline in wetland area. The future of the Te Harakiki wetland system is now more positive.</p>

The Hydrological Viability of Te Harakiki Wetland, Waikanae

<p>Wetlands are unique natural resources that play an important role in the hydrological cycle. There is a dynamic link between wetland hydrology and inputs from both surface and groundwater resources. Shallow groundwater abstraction near the Te Harakiki wetland at Waikanae has the potential to impact on the wetland' hydrosystem. To assess the likelihood of this occurring, a detailed analysis of recent changes, the hydrological regime, and the water balance of the Te Harakiki Wetland system was undertaken. The hydrological regime of the wetland system was assessed by various monitoring sites established around Te Harakiki to measure rainfall, soil moisture, surface and groundwater levels. Analysis of (decadal) historical aerial photographs allowed changes in spatial extent of the open water habitat (lagoon) and the urban area of Waikanae Beach. Comparisons were made between wetland extent, population increase and urban area expansion. These data, together with a simple water balance, and historical climatic records, were used to explain the drastic decrease in wetland extent. Climatic factors and goundwater are the major driving forces behind the wetland's hydrologic regime. The surface water outflow from the system is greater than the surface water inflow, but this may be affected by the tides. The surface and groundwater systems in the area are closely linked. They have similar responses to rainfall events. Groundwater abstraction in the area appears to have minimal impact on the water level within the wetland. The exact nature and extent of abstraction around the wetland is unknown. The reduction in flood pulsing as a result of channel modification, and the fragmentation of the area for the construction of the oxidation ponds are the likely explanation. The current restoration efforts in regard to controlling pest species and excluding stock from the wetland have halted the decline in wetland area. The future of the Te Harakiki wetland system is now more positive.</p>

Simulated range of mid-Holocene precipitation changes to extended lakes and wetlands over North Africa

Enhanced summer insolation over North Africa induced a monsoon precipitation increase during the mid-Holocene, about 6000 years ago, and led to a widespread expansion of lakes and wetlands in the present-day Sahara. This expansion of lakes and wetlands is documented in paleoenvironmental sediment records, but the spatially sparse and often discontinuous sediment records provide only a fragmentary picture. Former simulation studies prescribed either a small lake and wetland extent from reconstructions or focused on documented mega-lakes only to investigate their effect on the mid-Holocene climate. In contrast to these studies, we investigate the possible range of mid-Holocene precipitation changes in response to a small lake extent and a potential maximum lake and wetland extent. Results show that the maximum lake and wetland extent shift the North African rain belt about 3 ° farther northward than the small lake extent. Vegetated wetlands cause a larger precipitation increase than the equally-large lakes due to their high surface roughness. A moisture budget analysis reveals that both, lakes and wetlands, cause an enhanced inland moisture transport and local moisture recycling to their southern side. In contrast, increased moisture advection by the Harmattan winds causes a drying response to the north of the lakes and wetlands. These results indicate that the latitudinal position of the lakes and wetlands influences the northward extension of the African summer monsoon. In the sensitivity experiments, the northern position of West Saharan lakes and wetlands substantially contributes to the strong monsoon northward shift seen in the maximum lake and wetland simulations.

Development of the global dataset of Wetland Area and Dynamics for Methane Modeling (WAD2M)

Seasonal and interannual variations in global wetland area are a strong driver of fluctuations in global methane (CH4) emissions. Current maps of global wetland extent vary in their wetland definition, causing substantial disagreement between and large uncertainty in estimates of wetland methane emissions. To reconcile these differences for large-scale wetland CH4 modeling, we developed the global Wetland Area and Dynamics for Methane Modeling (WAD2M) version 1.0 dataset at a ∼ 25 km resolution at the Equator (0.25∘) at a monthly time step for 2000–2018. WAD2M combines a time series of surface inundation based on active and passive microwave remote sensing at a coarse resolution with six static datasets that discriminate inland waters, agriculture, shoreline, and non-inundated wetlands. We excluded all permanent water bodies (e.g., lakes, ponds, rivers, and reservoirs), coastal wetlands (e.g., mangroves and sea grasses), and rice paddies to only represent spatiotemporal patterns of inundated and non-inundated vegetated wetlands. Globally, WAD2M estimates the long-term maximum wetland area at 13.0×106 km2 (13.0 Mkm2), which can be divided into three categories: mean annual minimum of inundated and non-inundated wetlands at 3.5 Mkm2, seasonally inundated wetlands at 4.0 Mkm2 (mean annual maximum minus mean annual minimum), and intermittently inundated wetlands at 5.5 Mkm2 (long-term maximum minus mean annual maximum). WAD2M shows good spatial agreements with independent wetland inventories for major wetland complexes, i.e., the Amazon Basin lowlands and West Siberian lowlands, with Cohen's kappa coefficient of 0.54 and 0.70 respectively among multiple wetland products. By evaluating the temporal variation in WAD2M against modeled prognostic inundation (i.e., TOPMODEL) and satellite observations of inundation and soil moisture, we show that it adequately represents interannual variation as well as the effect of El Niño–Southern Oscillation on global wetland extent. This wetland extent dataset will improve estimates of wetland CH4 fluxes for global-scale land surface modeling. The dataset can be found at https://doi.org/10.5281/zenodo.3998454 (Zhang et al., 2020).

Influence of a small and maximum lake and wetland extent on the simulated West African monsoon precipitation during the mid-Holocene

&lt;p&gt;During the mid-Holocene, an expansion of vegetation, lakes and wetlands over North Africa reinforced the West African monsoon precipitation increase that was initiated by changes in the orbital forcing. Sedimentary records reflect these surface changes, however, they provide only limited spatial and temporal information about the size and distribution of mid-Holocene lakes and wetlands. Previous simulation studies that investigated the influence of mid-Holocene lakes and wetlands on the West African monsoon precipitation, prescribed either a small lake and wetland extent or focusing on mega-lakes only. In contrast to these simulation studies, we investigate the&lt;strong&gt; &lt;/strong&gt;range of simulated West African monsoon precipitation changes caused by a small and a potential maximum lake and wetland extent during the mid-Holocene.&lt;/p&gt;&lt;p&gt;Therefore, four mid-Holocene sensitivity experiments are conducted using the atmosphere model ICON-A and the land model JSBACH4 at 160 km resolution. The simulations have a 30-year evaluation period and only differ in their lake and wetland extent over North Africa: (1) pre-industrial lakes, (2) small lake extent, (3) maximum lake extent and (4) maximum wetland extent. The small lake extent is given by the reconstruction map of Hoelzmann et al. (1998) and the potential maximum lake and wetland extent is given by a model derived map of Tegen et al. (2002).&lt;/p&gt;&lt;p&gt;The simulation results reveal that the maximum lake extent shifts the Sahel precipitation threshold (&gt; 200 mm/year) about 3 &amp;#176; further northward than the small lake extent. The major precipitation differences between the small and maximum lake extent results from the lakes over the West Sahara. Additionally, the maximum wetland extent causes a stronger West African monsoon precipitation increase than the equally large maximum lake extent, particularly at higher latitudes.&lt;/p&gt;

Deglacial records of terrigenous organic matter accumulation off the Yukon and Amur rivers using lignin phenols and long-chain n-alkanes as biomarkers

&lt;p&gt;Anthropogenic climate change has profound impacts on Arctic temperatures, with consequences for Arctic ecosystems and landscapes, and the stability of organic-rich permafrost deposits. When mobilized, these permafrost deposits might release vast amounts of greenhouse gases. We use periods of past rapid warming in the high latitudes as analogues to study the ecological changes and effects on permafrost stability under climate change. We used marine sediment cores from the Bering and Okhotsk Sea continental margins, off the mouths of the Yukon and Amur rivers, to study two types of terrigenous biomarkers, which trace different terrestrial organic carbon (OC) components and transport pathways, and cover the early deglaciation to the early Holocene. The Yukon basin remains within the permafrost-affected region today, whereas the Amur basin changed from being subject to complete permafrost cover during the last glacial to permafrost-free conditions today.&amp;#160;&lt;/p&gt;&lt;p&gt;Vascular plant-derived lignin phenols were analyzed and compared to published n-alkane content data. The carbon- and sediment-normalized contents of the vanillyl phenols (V), syringyl phenols (S), and cinnamyl phenols (C) phenols (&amp;#923;8 and &amp;#931;8) reflect the content of lignin dominantly transported by river runoff. The C/V and S/V ratios serve to distinguish between woody and non-woody tissues of angiosperms and gymnosperms. The acid to aldehyde ratios of V and S phenols ((Ad/Al)&lt;sub&gt;V&lt;/sub&gt; and (Ad/Al)&lt;sub&gt;S&lt;/sub&gt;) indicate the degree of lignin degradation. In addition, the ratio of 3,5-dihydroxybenzoic acid to V (3,5Bd/V) likely reflects the wetland extent, while lignin reflects primarily transportation into the marine sediment via surface runoff. In contrast, the n-alkane contents represent primarily terrigenous organic matter eroded from deeper deposits and a second marker for wetland extent via the Paq index. Lignin and n-alkane mass accumulation rates (MAR) can thus be used to reconstruct the mobilization of different carbon pools and the relative timing of the processes leading to their export to the ocean.&lt;/p&gt;&lt;p&gt;The MAR of biomarkers and the wetland indicators 3,5 Bd/V and Paq start to increase in the Bering Sea sediment during the early deglaciation (19-14.6 ka BP), while no obvious change in lignin MAR in the Okhotsk Sea occurred during this time. We observe distinct peaks of mass accumulation rates, wetland indices and indicators for degradation of lignin (Ad/Al) in both sediment cores during the warm B&amp;#248;lling-Aller&amp;#248;d (12.9-14.6 ka BP) and Pre-Boreal (9-11.5 ka BP) intervals, and during the Younger Dryas cold spell (11.5-12.9 ka BP). In contrast, in the Okhotsk Sea, the ratios of S/V and C/V did not change before the Preboreal.&amp;#160;&lt;/p&gt;&lt;p&gt;Our biomarker data suggest that the permafrost in the Yukon basin may have started to be remobilized by inland warming leading to wetland development in the early deglaciation, while the onset of permafrost degradation in the Amur basin occurred during the Preboreal.&lt;/p&gt;

Development of a global dataset of Wetland Area and Dynamics for Methane Modeling (WAD2M)

Seasonal and interannual variations in global wetland area is a strong driver of fluctuations in global methane (CH4) emissions. Current maps of global wetland extent vary with wetland definition, causing substantial disagreement and large uncertainty in estimates of wetland methane emissions. To reconcile these differences for large-scale wetland CH4 modeling, we developed a global Wetland Area and Dynamics for Methane Modeling (WAD2M) dataset at ~25 km resolution at equator (0.25 arc-degree) at monthly time-step for 2000–2018. WAD2M combines a time series of surface inundation based on active and passive microwave remote sensing at coarse resolution (~25 km) with six static datasets that discriminate inland waters, agriculture, shoreline, and non-inundated wetlands. We exclude all permanent water bodies (e.g. lakes, ponds, rivers, and reservoirs), coastal wetlands (e.g., mangroves and sea grasses), and rice paddies to only represent spatiotemporal patterns of inundated and non-inundated vegetated wetlands. Globally, WAD2M estimates the long-term maximum wetland area at 13.0 million km2 (Mkm2), which can be separated into three categories: mean annual minimum of inundated and non-inundated wetlands at 3.5 Mkm2, seasonally inundated wetlands at 4.0 Mkm2 (mean annual maximum minus mean annual minimum), and intermittently inundated wetlands at 5.5 Mkm2 (long-term maximum minus mean annual maximum). WAD2M has good spatial agreements with independent wetland inventories for major wetland complexes, i.e., the Amazon Lowland Basin and West Siberian Lowlands, with high Cohen's kappa coefficient of 0.54 and 0.70 respectively among multiple wetlands products. By evaluating the temporal variation of WAD2M against modeled prognostic inundation (i.e., TOPMODEL) and satellite observations of inundation and soil moisture, we show that it adequately represents interannual variation as well as the effect of El Niño-Southern Oscillation on global wetland extent. This wetland extent dataset will improve estimates of wetland CH4 fluxes for global-scale land surface modeling. The dataset can be found at http://doi.org/10.5281/zenodo.3998454 (Zhang et al., 2020).