## 1 Introduction  
Land use change induced by human activities is a major source of greenhouse gases (GHGs). [PERSON] [1] estimated that about 1/3 of carbon emissions released to the atmosphere since 1850 has resulted from land use change. [PERSON] and [PERSON] [2] estimated that the average annual rate of deforestation was about 4.25 MH during the time period of 1850-1990. The annual rate of deforestation has increased to 8.3 MH in 1990s and then decreased to 5.2 MH during the past decade [3]. Expansion in cropland is the major source of land conversion and deforestation. Traditionally, the expansion in cropland has occurred to satisfy the need for higher demands for food and fiber products. During the past decades several countries around the world have launched biofuel programs to produce renewable fuels from agricultural resources. Several papers have assessed the economic and environmental impacts of these programs. The early papers published in this area suggested that the USA corn ethanol program could cause major land use implications [4; 5; 6]. However, the more recent studies find that the early estimates have overstated the land use implications of this program [7; 8; 9; 10; 11; 12; 13]. While research studies in this area have distinguished and examined the important factors which determine the land use impacts of biofuels and their geographical distributions no attempt has been made to validate the land use estimates due to biofuels in the face of actual observations [14]. The reason is simple. Prior to the last couple of years, there was insufficient data on global land use change during the biofuels boom era. However, now we have that data, and it can be used to better calibrate prior estimates of land use change, which is the objective of this paper. The global biofuel programs, particularly the USA and EU mandates, took off in the early 2000s. However, prior to the past 5-6 years the level of biofuel production was very low and there was no way to get any idea of land use changes that might come about due to the much higher mandated levels of biofuels. In 2011, USA corn ethanol production was over 14 billion gallons, near the Renewable Fuel Standard (RFS) level of 15 billion gallons stipulated for 2015. In this year Brazil also produced about 6 billion gallons of sugarcane ethanol, and the EU members jointly produced more than 4 billion gallons of ethanol equivalent of biofuels (including ethanol and biodiesel). Thus, with these large magnitudes of biofuel production we should be able to see some impacts of land use change even if we still cannot isolate the biofuels induced part of that change with precision. The existing estimates for the indirect land use change (iLUC) emissions due to biofuels are usually obtained from economic partial or general equilibrium models. To estimate iLUC emissions economic models, one way or another, estimate induced land use changes due to biofuel production. A land supply system which relates supply of different land types to their return or their land conversion costs is a key and common component of economic models used in this area. The existing Computable General Equilibrium (CGE) models usually use Constant Elasticity of Transformation (CET) functional forms to define their land supply system. Land transformation elasticities are needed to define a land supply system. These elasticities are difficult to directly estimate using econometric methods due to lack of sufficient quality data. In some circumstances the land transformation elasticities can be retrieved from the exiting land supply elasticity estimates [15] or can be estimated using simulated pseudo data [16]. A calibration or tuning practice is an alternative method which can be applied to tune land transformation elasticities for large and global CGE models [13]. In this paper we use observed information to tune the land transformation elasticities for the GTAP-BIO model.